I revive this thread : in my search engine (DuckDuckGo), with keywords "Civ6" "single" "team" "how to", it arrives in 4th position! It means that there are still many people looking for an answer, and very few answers...
Even after September 2019 patch, GS add-on, version 1.0.0.341 (443561) : YES, in the single player settings, you can still see a box allowing or not a team,
but NO, you still can't select/create a team. Only in multi-player modes as Swissy suggests above.
If you have a weak network, or unstable (locally or in your country), it is not advisable to play a lengthy game alone in multiplayer mode with the use of a network ressource (ex. Lan): if you go in Steam "offline mode", or your network/Lan has troubles (or your firewall), you can crash anytime in the game.
For better comfort and stability, it is preferable to play a game using Hot Seat (all needed files stay on the same computer, no network activity)
Now, the best, for comfort and stability, is to play it in a Single game environment. It is much simpler that it sounds :
- PREPARE a game using the multi-player screen (c.f. Swissy): it is quite lengthy, and you need to setup leaders 1 by 1, without forgetting to assign a team number to your own team, a difficulty for each leader, and perhaps changing some flags/colours. For avoiding conflicts, be sure to remove all settings that are multiplayer related : by instance, don't forget to change the speed (from on-line speed to something useful in single. I always play epic or marathon)
- then launch this game in multiplayer as if you are going to play it.
- When the game starts, save it right away, with a clear name, ex. : "Team SEONDEOK GORGO Deity.Civ6Save". Better to close Civ6 for avoiding some troubles with Alt+Tab and/or memory leaks.
- go to Explorer, and where your saved games are stored. For me it is : "...\Documents\My Games\Sid Meier's Civilization VI\Saves\Multi", locate your saved game
- paste it into the folder for single games : "...\Documents\My Games\Sid Meier's Civilization VI\Saves\Single"
- start Civ6, go to Single, Load, select the file you just pasted, and launch the game as it is.
- now, as it is a saved game, it will always start the same way: just open the menu and use the RESTART option for running a new game with your settings (map settings, same leaders, team, but new seeds and locations) until you find a nice starting location. The saved game becomes a generic team game (with pre-set partners/opponents)
If you wish to try without having to do the annoying preparation, here is a game I started.
Just put it into your single game folder. Unfortunately, if you can change the starting map/locations by restarting, you can not change the leaders nor the map configurations
It is : Seondeok (science) + Gorgo (culture + reliable), Deity, large, Fractal, Epic speed, sparse resources (never sparse enough to my tastes...), all vics enabled, random map layout (weather, sea, age), 4 random events, 18 city states. Versus a large bunch of uglies I like to crunch. Very very subjective: I have added leaders who were very annoying in many of my previous games, just a revenge, nothing more.
Beware: the saved game can still be stored under the MULTI folder though.
Forgot: it is for the GS add-on, and with 2 mods allowing City States to survive much longer than usual (at high levels, leaders just steal the show early by capturing City States).:
"Stop Steamrolling City States" + "TCS free wall for City States" : CS will start with many more units, and walls. The 2 mods are very good, stable, accessible and updated for GS in the workshop.
I also use the mod "Multiplayer Restart Button", very useful for allowing several "restart" with a game designed under multiplayer environment.