Although it was possible to fill every gun position, most times we'd be lucky to get one or two gunners, especially as they had to join while the bomber was still on the ground. A message would go out that "[CPID] is crewing a B17 at field [field name]"and we'd usually wait a few minutes to see if anyone turned up. I think the broadcast was automatic but could be turned off or repeated.
I often went as a gunner, most memorably for my squad CO, Spiff. He would get into turn fights with me in the chin gun. I learnt a lot about gunning from Spiff and we had many fun flights together.
After I became a Game Assistant, (AWARHelen, again taught by Spiff!) I had to go into all of the arenas, including the RR arenas which I'd always avoided until then. I then started going in them sometimes just for a change and found that fully gunned B17s were much more the norm there - as long as everyone's connection held out. They would take them to an enemy field and often spend time vulching the spawn point, or go inside the hangars and destroy them with the guns from the inside! Not exactly historical but a lot of fun.