COD 5 WAW multiplayer party questions

Hello,
I don't know if any one here on sythe has much knowledge about this
but in COD 5 you can invite friends to your party so you can play as a team with them.
I do this very often with a friend of mine it all goes well etc...
But lately a few friends also purchased cod waw and one of them has a problem
if I invite him to my multiplayer party he gets the error:
Unable to join game session
or this error:
Game session no longer available

The things we tryed to fix this:
-restart ps3
-restart internet
-friend inviting me

Also if I'm in a game lobby I can invite him but the problem then is the lobbys fill up so unbeleveble fast. So most of the time he gets the error that the game is full

If any one knows how to fix this please let me know

thanks in advance