There's no way to perform in-game text chat. If you're on PC, you can activate the UPlay Overlay using Shift+F2, and chat with your friends. Of course, that means that the players you are playing against have to be your friends. (go and friend everyone, right now! :P)
Alternatively you can just use the in-game voice chat. I haven't encountered anyone who used it though, even in the prequel AC:Revelations.
Personally I think this is a huge problem with playing cooperatively on Multiplayer, esp. when people start doing the darndest things in Wolfpack. On the other hand, if there were in-game chat, you can imagine tensions would be running pretty high.