As LouLong said. If you have met a human player then all diplomacy is considered 'on' although you may not be able to actually sign RoP, trade or alliances until you have the right techs. AFAIK, however, it is still ok to gang up on another civ, even if you don't have an embassy, even if you have not even discovered Writing.
Before you find a human player, you are not supposed to divulge any information that could assist you or another player directly (such as, "I'm in the NW corner of the map and am at war with the Germans, Hittites and Maya, please help!"). It is ok to complain about starting position, stupid moves you made, the obnoxious barbarians, etc. as that is (usually) idle chatter.
Finally, you are not allowed to make diplomatic agreements (I won't attack you, I promise, just please don't beat the piss out of me!) when you are still at war. It's ok to end the war, but if you are allied with the other human player, you are limited to the diplomacy options in the game (i.e. end war or no diplomacy). You can, however, agree to diplomatic options not on the table with AI civs (such as, "I will give you silks and furs if you end the war").
BTW, sent turn on to Pedders