Welcome to the Ofpec Forums. I'm at work without access to ArmA, so this explanation is from memory only. But you have my permission to shoot me in any MP game of your choice if it doesn't work
1) Press [SPACEBAR] an hold it for second or two.
2) You should now recieve the option to select TEAM, as far as I remember.
3) Or you can try to select the team you're after using the numerical keys. Use 9-1 for Team Red, 9-2 for Team Green. I havent tried that in ArmA2, but it worked in both predecessors, som I'm quite sure it's still there.
And to make this thread a bit more editing related (wich is actually required here at Ofpec), you may also define the groups frpm start by placing this in the initfield of any unit:this assignTeam "RED"
The unit in question will then be marked oiff as a member of team Red upon mission start.
Once more: Welcome to Ofpec