When this happens, it's usually 3 things:
1-The party is full and no one else can join.
2. The game is in progress and both teams are full.
3. The game settings have changed. Sometimes, you'll get a little message that says the game settings have changed and you need to restart to update it. The things is, even if you update yours and your friends dont, you cant join them until they update it as well. So, make sure you're all on the same updated settings.