If you have, say, 3 teams, named team1, team2, and team3, and you do this command:
/scoreboard teams join team2 @p
you will join the team "team2". However, if you do this:
/testfor @a[team=!team1,team=!team2]
it will not "find" you like it should.

This also affects other commands.

If I do
/tellraw @a[team=!team1,team=!team3] "test"

then it will not show for anyone.