When in the game then sometimes it is not clear why you can't invite friends into your group. When friends are still in a different area then you can't invite them to join. For example. If I am about to start the end game and my friend is still in the BOO or running other missions, then I can't invite him. It is unclear why I can't invite him. Experience told me we need to be in the same part of the game. (like in div1, where you can't invite friends who are running in the underground when you yourself are in the terminal.).
It is unclear WHY you can't invite him. It would be nice you the game player would get a shot notification why an invite is unsuccessful. What does the other player have to to (like level up, or go to a specific game erea), in order to be able to join the group.