It's not a big problem because I can join myself the game where my friends are, but I'm bored at work and I can't play Crysis 2 90% of times somebody invites me to a game I can't accept the invitation, I get the message "an error has occured" (or similar), and often the server is not full. The only way I can usually join a squad is when the person that invites me is in the menus.

Any ideas?

You can always go to the server browser and filter the server by friends!