So....
1. A way to tell that a gunner is attached to you
2. Ability to kick naughty gunners
+1 Please please please!
3. Message announcing the departure of gunner (weather intentional or not).
+1 I overall feel the ".join" feature is one that can be relatively easily but immensley expanded in the game, but highest priority would be to allow the PIC more authority over their aircraft and passengers. If a PIC abuses it, say kicking out their helpful gunner before landing, I doubt it won't be long before anybody decides to join and assist that pilot again..... abusable, but only until you run out of the unweary and trusting (and most those players get broken-in with the Alt+F4 joke anyways).
+1 for message that your gunner has left
+1 for ability to kick gunner
good wish (and easy to coad) 
It's an assumption, but a pretty safe one.... still, even the easy can be impossible without the time to do it.
Sounds reasonable.
Until then, don't let "just anybody" in your plane with you. 
That is my rule of thumb... but... this is a game... and honestly me saying "no, you can't come along/watch and enjoy the fun" seems kinda wrong if not shallow.