Once you've booted up your copy of Portal 2 PS3, a simple press on the Select button is all it takes to bring up the Steam overlay. If you're familiar with Steam already, the PS3 overlay will look very familiar.
The PS3 overlay has access to chat, Portal 2 news, your friends list, and achievements. Doing actions like linking your PSN ID to a Steam account, sending out invites, and starting chats are just a couple of button presses away.
There are a few minor restrictions to note: Portal 2 trophies can unlock their corresponding Steam equivalent but not the other way around. Also, PS3 Steam Cloud saves can't be used with PC versions of Portal 2.