Friend Cannot Join Multiplayer Sessions

[KlassicKanadian]

Recently bought the game for our friend and we've been trying to get him into multiplayer. However when we invite him into our lobby using the steam friends list, it says that there's an error in joining the match. He can't use the invite code feature because the join using invite code button doesn't show up for him. He also cannot see our public matches either. Is there a level or hour requirement to play multiplayer or something?

[Massy_Pal]

I'm having a similar problem where I've always been able to play the game yet cannot seem to join my friends games. We've figured out that we are running different versions of the game, him being on the latest v21 when for some reason I'm on v17. After going through the usual suspect (verifying the files integrity and performing a clean install of the game) I cannot seem to get steam to install the actual latest version of the game which is fairly infuriating. Does anyone know how I can force Steam to perform the update to v21 ?