this might help, I don't have the game to try it though.
Seems complicated and the English is a bit broken but w/e
http://forums.gameaxis.com/showthread.php?t=1508344
Many new proud owners of RE5 eh?
wana play co-op witha friend but have no idea how to do it?
RE5 doesn't have the most convenient of means to invite friends to play.
but it's not impossible either.
Start RE5, create a invite only game session,
In ur XMB (PS Home button) - the PS3 OS.
go to the friend u wanna invite and press triangle.
That pulls out more details.
first option: Invite this Player.
Now for the person who got invited, u can't find a invite link or link etc.
u receive a message which has the game session burnt into the message's backend (no one can see it).
Open XMB, select eitehr urself or any other friend in the friends list.
Triangle and hit Accept Invitation.
Then, it'll search ur received messages for invites.
Select the invite u wanna join and join it.
Voila! CO-OP play!
Proud Sony Rear Admiral
