Not sure if this will be useful, but if you are having the issue where your club loses its 7th and 8th members, then I have found that opening and closing the panel for changing the clubs outfits seems to reset things in my game - the two spaces will become unlocked again as soon as I close that window.
I have used this a couple of times today in order to get a gathering of an entire 8 sim club. Unfortunately it's just a temporary fix, only lasting until the next loading screen, so you do have to get to the lot you want to visit before doing this and starting a gathering.