Read this:
http://www.bioniccommando.com/forum/showthread.php?p=14667

If you play a RANKED team deathmatch or CTF game with a friend, you will end up on opposite teams.  I don't even know how this was allowed to pass certification.  Even worse is the fact that if you play a private game,  you are the host and you can't search for games.  You can only invite other players, and those are the only people you're able to play with. 

It's a shame, too, as the multiplayer in this game is tons of fun.  Hopefully it will be quickly fixed.  If you're not happy with team deathmatch/CTF as it currently is, I suggest you register at the official forums and let your voice be heard.  Just try not to sound too rude about it.  More flies with sugar than with vinegar, you know.

I dont get you.  I dont recall many games that let you enter into ranked matches with friends, there are reasons behind that.  Seems like being on opposite teams is the best you could hope for.  Also, aren't private games for that purpose? That you host and only invite friends?  Otherwise would it not be a public game?

You're not supposed to be able to search for private matches. That's why they're PRIVATE.