first of all, i think it might have something to do with your rank compared to theirs. If your's is too high you can't join them or you'll be too overpowered for the other team.
same thing if they're higher than you, you'll be too underpowered for the other team.
I honestly don't know how to play online on brink yet, i think its under "free play" but everytime i do that i can open the leaderboard which pauses the game, meaning there are no real people in it.
but i managed to get in one the other day somehow, but i dont know how.
Thus, you should wait a couple of days for people to level up before trying to go online again.
I mean, i'm rank 5 and i've been rank 5 since yesterday, i haven't played online since rank 2.
so what i'm doing is waiting until a good majority of players hit rank 5.