When I was living and working in Poland, I helped promote events for foreigners living there. It was a rather easy sell, as there weren’t too many at the time in my part of the country (I wasn’t in Warsaw), and the network was targeted enough that word spread quickly without the use of the internet. These events happened usually every two weeks, and it didn’t take too long for it to get a little boring, as the same folks usually showed up (and same activities took place).
A large number of the foreigners were English teachers, and in conversation with them I learned that their students were studying so they could work in the UK. An idea clicked in my head, and I said that we could open up our meetups to Polish people who are looking to travel/work abroad, as all of us regulars had done, thus offering a shared experience and something “educational” from the event.
It’s just one example, but I think that the strengths within a group can be leveraged to attract new audiences by finding how to give something more than just the event in question.
Hope that helps!