Two-player co-op has arrived in XCOM 2 [official site] with the first beta release of a mod which hopes to one day support twelve-player co-op. Two seems a good number to start with, though. You have someone to bounce ideas off, someone for moral support and encouragement, someone to plan and scheme with, and someone to blame when snakes are choking out your champ.
This first public release lets two players team up, with one player hosting and the other able to bring in characters from their own campaign. Each controls their own characters. At the end of a mission, the secondary player will return to their own campaign with any XP and loot they earned. Because of how the mod is built, missions with turn timers may be wonky - which is a fair chunk of XCOM 2.
It sounds fairly simple but hey, I enjoyed playing Dawn of War 2 co-op with its similar unit-splitting way.
Team Dragonpunk have big plans, mind. They want to make 12-player co-op and say they "are waiting on Firaxis to approve code" that would enable it. Beyond that, they're planning to make a whopping great total conversion named Dragonpunk. It sounds a bit Shadowrun-y, mixing cyberpunk with dragons and metahumans and magic and whatnot. They're trying to fund that through GoFundMe.