Civ5 MP war depends heavily on computer preformance, and my laptop just doesn't cut it at the causal LANs I have with my friends, so I've developed a few "Support" strategies where I take a backseat to war and attempt to support a teammate who does the fighting for me.
Babylon
This is an obvious civ for support when your team shares tech. GS bulbing military techs in the early game can give your ally a frankly overpowered advantage over the enemy. The play isn't isn't much different from SP Babylon, except for grabbing a good deal of Baby Walls (I call them "cribs") and prioritizing the Kremlin/Himeji/GW (making sure to get at least two of them). That along with Oligarchy and some CS exp farming (when you dec war on a CS just to farm up exp from bombards) on my ranged units can keep you safe from aggression. The cookie-cutter GL->Theo->Hagia->(optional lib finisher GS for Edu) PT->Astro fast tech build really isn't as viable in MP, as it sacrifices military techs early on---that is, unless your ally is Bismark >
. Then bombing for Civil Service/upper tech tree pays off huge!
Other notes would be that I go into Trad for this build to help rush the GL and for Oligarchy (as mentioned before) and finish it so I can make a tall science empire.
One of the biggest priorities is getting Artillery first and making sure the game doesn't go to nukes...both of those can crack your cities no matter how much def you have
, but those are general MP tips.
Bismark/Nebby has been houseruled out because of how effective it is. I supposed an Egypt/Rome or Russia build might counter it, however, with a GL deny and enough swords to punch through the Landk's.
(also, Japan/Nebby is outlawed because of using that early GS in a steel cascade
it is terrifying to see 60 turn Samurai)
Greece
Being a CS whore is a nice way to support, especially with Educated Elite. Usually I just go Lib->some Patronage->Rat, using the GS from Lib to bulb Steel or something (a note: with TWO players who can both farm GS's, coordinated GS/Oxford cascades can be HUGE. I think the next thing that will be houseruled out will be bulbing in general because of this). I do, however, produce quite a few Comp Cav and Hoplites in the early game and try to use them or gift them to my ally. They are kind of of hard to screw up with, being so strong in the early ages. We often play with third party AI's or AI's on our teams and sometimes I use the unique unites to knock out the AI's (who, thank god, don't have simultaneous turns with us).
Semi-wide can generate a good deal of TR gold to support the CS whoring, and as mentioned with Babylon, turtle wonders are things I focus on (I'm a notorious turtler amongst the group xD).
One cool thing I did with Greece in the latest patch was pick up 2pts into Order for United Front and then fed my ally the units the CS's fed me! I was a middle man with a sizeable navy escorting gifted unit to be regifted in his territory.
Another time one of the guys counter-picked me with Genghis. He figured he would just kill off any CS I bought. However, one turn before he killed two of my CS's I dec'd war on the CS, which made his aggression "unwarranted" and the CS's united and declared perm war on him
. I just spam-clicked the dec war thing in-between turns, he couldn't avoid it because it was simultaneous with his attack.
Greece, overall, I would say is a decent support. Alex actually makes a really good primary warmonger as you can feed him gold to get CS's on your side.
Arabia
I would consider Harun the BEST support. Arabia is flexible, capable of generating huge amounts of gold to gift, produces 2x the luxuries, AND is good at the only kind of fighting I can participate in easily, which is air raids.
I go semi-wide to wide with Harun, doing Lib/Comm/some Rat/Atoc. I focus on science (which is pretty much a given for any support) and try to take advantage of "policy splitting" which is when players on a team specialize into roles by diving into different policies. It's very cool when one civ spent all of the SP's on Hon, but the other civ has Comm/Big Ben/Atoc and is buying/gifting the war civ fresh troops every turn.
Another trick we do with Arabia is to gift me cities after conquering them. Once puppeted my ally trades them to me, who buys out the courthouse &
buildings and adds it to my huge TR network. In general, trading cities around to abuse puppeting mechanics is something every support civ should do. It's feels very naughty when I'm in the backseat sitting at -17 happy holding onto razing cities and the conquering civ is at +20 happy.
Talking about happy, the thing that makes Harun tier 1 minus is the Bazaar and gifting lux's to the ally. It solves happiness problems, allowing the primary civ to expand or conquer at will. Arabia/Persia city trading is excellent because of this. Persia gifts all of the conquered cities (or cities that THEY founded) to Arabia, who then doubles the luxuries, eats the unhappiness, and gifts back the luxuries to Persia, who sits on 50+ happy a turn for the entire game, knocking the #$%^ out of the enemy during the many GA's. Arabia can also spam GE's (which, in all honesty, is kind of a unique GP to spam) to gift to Persia for GA's or to wonder rush, denying the enemy strategically--it's actually not that hard to read what wonders the other guys are going to want.
Egypt
This is a theoretically good support. It would work the same way as Arabia, generating happiness with BT's when gifted puppets and denying wonders. However, I overall think Arabia is better. Egypt only really shines early game with an Aristocracy TGL move.
Russia
Might as well be a primary civ, but it can shine like Arabia but with strat resources. Russia/Rome or Japan is a combo I've always wanted to try out. Fielding craptons of unique Iron units would be cool.
I don't own all the DLCs, so there might be other civ's I haven't played with which would make good supports (Korea comes to mind). I'm interested to see what other combos the community has used/thought of and what works well in games without AI's to knock over. I'm actually curious to see if anyone else even considers civilization specialization to be a useful thing in MP when both players can fight effectively. This may depend on the map, however, as I feel specializing generally gives a boost to science for that team, and an upgrade advantage matters a lot more in a choke as opposed to a wide front where #'s may be the deciding factor.
Babylon
This is an obvious civ for support when your team shares tech. GS bulbing military techs in the early game can give your ally a frankly overpowered advantage over the enemy. The play isn't isn't much different from SP Babylon, except for grabbing a good deal of Baby Walls (I call them "cribs") and prioritizing the Kremlin/Himeji/GW (making sure to get at least two of them). That along with Oligarchy and some CS exp farming (when you dec war on a CS just to farm up exp from bombards) on my ranged units can keep you safe from aggression. The cookie-cutter GL->Theo->Hagia->(optional lib finisher GS for Edu) PT->Astro fast tech build really isn't as viable in MP, as it sacrifices military techs early on---that is, unless your ally is Bismark >

Other notes would be that I go into Trad for this build to help rush the GL and for Oligarchy (as mentioned before) and finish it so I can make a tall science empire.
One of the biggest priorities is getting Artillery first and making sure the game doesn't go to nukes...both of those can crack your cities no matter how much def you have

Bismark/Nebby has been houseruled out because of how effective it is. I supposed an Egypt/Rome or Russia build might counter it, however, with a GL deny and enough swords to punch through the Landk's.
(also, Japan/Nebby is outlawed because of using that early GS in a steel cascade


Greece
Being a CS whore is a nice way to support, especially with Educated Elite. Usually I just go Lib->some Patronage->Rat, using the GS from Lib to bulb Steel or something (a note: with TWO players who can both farm GS's, coordinated GS/Oxford cascades can be HUGE. I think the next thing that will be houseruled out will be bulbing in general because of this). I do, however, produce quite a few Comp Cav and Hoplites in the early game and try to use them or gift them to my ally. They are kind of of hard to screw up with, being so strong in the early ages. We often play with third party AI's or AI's on our teams and sometimes I use the unique unites to knock out the AI's (who, thank god, don't have simultaneous turns with us).
Semi-wide can generate a good deal of TR gold to support the CS whoring, and as mentioned with Babylon, turtle wonders are things I focus on (I'm a notorious turtler amongst the group xD).
One cool thing I did with Greece in the latest patch was pick up 2pts into Order for United Front and then fed my ally the units the CS's fed me! I was a middle man with a sizeable navy escorting gifted unit to be regifted in his territory.
Another time one of the guys counter-picked me with Genghis. He figured he would just kill off any CS I bought. However, one turn before he killed two of my CS's I dec'd war on the CS, which made his aggression "unwarranted" and the CS's united and declared perm war on him

Greece, overall, I would say is a decent support. Alex actually makes a really good primary warmonger as you can feed him gold to get CS's on your side.
Arabia
I would consider Harun the BEST support. Arabia is flexible, capable of generating huge amounts of gold to gift, produces 2x the luxuries, AND is good at the only kind of fighting I can participate in easily, which is air raids.
I go semi-wide to wide with Harun, doing Lib/Comm/some Rat/Atoc. I focus on science (which is pretty much a given for any support) and try to take advantage of "policy splitting" which is when players on a team specialize into roles by diving into different policies. It's very cool when one civ spent all of the SP's on Hon, but the other civ has Comm/Big Ben/Atoc and is buying/gifting the war civ fresh troops every turn.
Another trick we do with Arabia is to gift me cities after conquering them. Once puppeted my ally trades them to me, who buys out the courthouse &

Talking about happy, the thing that makes Harun tier 1 minus is the Bazaar and gifting lux's to the ally. It solves happiness problems, allowing the primary civ to expand or conquer at will. Arabia/Persia city trading is excellent because of this. Persia gifts all of the conquered cities (or cities that THEY founded) to Arabia, who then doubles the luxuries, eats the unhappiness, and gifts back the luxuries to Persia, who sits on 50+ happy a turn for the entire game, knocking the #$%^ out of the enemy during the many GA's. Arabia can also spam GE's (which, in all honesty, is kind of a unique GP to spam) to gift to Persia for GA's or to wonder rush, denying the enemy strategically--it's actually not that hard to read what wonders the other guys are going to want.
Egypt
This is a theoretically good support. It would work the same way as Arabia, generating happiness with BT's when gifted puppets and denying wonders. However, I overall think Arabia is better. Egypt only really shines early game with an Aristocracy TGL move.
Russia
Might as well be a primary civ, but it can shine like Arabia but with strat resources. Russia/Rome or Japan is a combo I've always wanted to try out. Fielding craptons of unique Iron units would be cool.
I don't own all the DLCs, so there might be other civ's I haven't played with which would make good supports (Korea comes to mind). I'm interested to see what other combos the community has used/thought of and what works well in games without AI's to knock over. I'm actually curious to see if anyone else even considers civilization specialization to be a useful thing in MP when both players can fight effectively. This may depend on the map, however, as I feel specializing generally gives a boost to science for that team, and an upgrade advantage matters a lot more in a choke as opposed to a wide front where #'s may be the deciding factor.