My theater audience wasn't limited to people we already knew. When a show was in rehearsal, I'd talk about it to anyone I met, even perfect strangers. If they seemed interested, I'd invite them, and nine times out of ten they'd come to it.
Once it got going, people used to stop me in the street and introduce themselves and ask to be put on my invitation list. They'd tell me they knew someone who was in one of the shows, and didn't want to be excluded from future shows if that person wasn't involved.
At that point I probably could have changed over to conventional publicity. If people know it's going to be a popular event, they'll go to it for that reason. If not, you've got to give them some other reason, and you've got to make them feel wanted.