"Yaster, part of SCEE's community team, has detailed the PS3's new Invite System. It's pretty slick, especially if you're a regular user of the text chat facility built into the PS3's XMB.
How does it work? It's simple: get into a Chat Room, load up a supported game and click the relevant option (for Bioshock 2, this is "Manage Invites". From there, select the empty box at the top as normal."