If you look at your friends list to see which friends are "Joinable" - it seems that only the actual HOST of a GRAW game reports the joinable status correctly. If you choose someone who is just a member of a game, they will show joinable, even if the game is full.
Example - 2 brothers playing graw, same game (I know they play together 100%). One shows joinable and one does not. If you try to join the joinable one, server is full. Each game, one of them shows joinable, and the other doesn't (the host). Unless the actual host shows joinable, you cant join..and there is no way to determine which one is hosting.
If I'm looking at this wrong, let me know.