Civilization 6 is out and it is exceptionally good. One thing it's missing, however, is an option for team multiplayer. But it looks like the feature is coming, based on the findings of this Reddit thread, and more interestingly for the daring (or reckless) among you, there is a way to enable it, to a limited extent, right now.
A number of references to team multiplayer have been found in the game's code, along with a comment in the XML stating, "Important: Disabling team pulldowns until day 0 patch." The general consensus is that the feature was planned and then kiboshed, probably due to some bug that reared its ugly head at the last minute. But you can re-enable it, as instructed in the Civ Fanatics forum, by editing line 1134 ("playerEntry.TeamPullDown:SetHide") in "C:\Program Files (x86)\Steam\steamapps\common\Sid Meier's Civilization VI\Base\Assets\UI\FrontEnd\Multiplayer\StagingRoom.lua" (or whatever path you use) from "true" to "false."
Even though you can turn it on, you may not want to rush to do so, since team multiplayer clearly hasn't been fully (or even mostly) implemented. According to follow-up comments, civilizations can't share research or technology, and while teammates can see one another's starting locations, areas of the map uncovered after the beginning of the game are not shared. Victory conditions aren't shared either, so it's impossible to achieve a team victory.
And of course there's always the risk of crashes that can so easily arise from this kind of horsing around. "At the moment Team Multiplayer is not a feature supported in Civilization 6," a 2K rep said. "If someone is going to mod any of the game files or XML files to change how features work, this is obviously at their own risk, so always backup the files they’re planning to mod beforehand."
I think the "at their own risk" part bears repeating.
Civ 6, as we noted here, is "very moddable," but even so this sounds like one that you might want to stay away from, especially since official team multiplayer support (hopefully in the relatively near future) looks like a sure thing. I won't judge if you just can't wait, though—and if you do give it a try, let us know how it goes in the comments.