@Beardedgeek Before we were given platforms in the game, my Global Superstar has had this problem. He's hardly ever 'alone' on the stage, in fact. I've had to take measures, such as locking the stage door for everyone but Erik, but unfortunately, this isn't a fail-safe. Since on the other side of this venue is the only bathroom, I have made the mistake, trying to be nice, by not locking that door. This is what I ended up with just the other day. I am inclined to 'fix' this situation by doing a very slight remodel on what I think is supposed to be a 'dressing' room. I'll add another door that blocks Sims from entering the stage.
https://i.imgur.com/i1OhzwG.jpg
In some cases I've been able to use the rope line to keep out everybody but my Rock Star, (unless, of course Judith shows up). But, it is and has been an ongoing problem, at least for me.
