The reply above is correct.
I had this very same problem, I shall provide how it happened to me and how you can fix the issue.
- I started a group flight
- I invited my friend and started the game
- I left the game and group before my friend
- To fix, your friend must make a group flight and invite you
- Then, you must quit to main menu and choose to leave group
- Now your game will be FIXED!
If your friend started the first group flight, then go through this list the same but vice versa.