I'm trying to join a friend through an invite they sent me through Steam's messaging system but when I try to join, I get an error that reads:

"The match you're trying to join is running on an incompatible version of the game. Please try upgrading your gamer version."

I don't understand why this is happening. Me and the other two are using the same version of the game, so we should be playing together. If anyone has any explanation or solution for this, I would appreciate it.