"Teaming up" with another player in a FFA game after it has already started. This is different than teaming up BEFORE a game starts because teaming AFTER a game starts gives the 'team' two ffa teams - twice as many units as every other team in the game. Teaming is generally considered bad form as the teamed 'team' has a huge advantage over the other individual teams.
In Myth SB, 'teaming' is one of the official game options - via the 'allies' feature where people can team with each other during a game. The difference in SB is that when teams ally with each other the fact is announced to other players who can see who is allied with who.
Allies is not a very popular option in Myth 2.