If you want an idea that isn't trying to persuade all your friends to not have boyfriends, you could get a whiteboard so you can write notices to each other including if you're expecting someone round. It can be more convenient that trying to figure out if you've told everyone.
I think that people tend to grow out of this way of thinking as they get older. But I'm not sure because I never talk to any of my friends about this stuff these days and they don't bring it up either. After all, the crush is their own person and not a thing to fight over.