I looked into how groups work a long time back. There's something there, though I'm not sure it's in my grasp.
Steve Randy Waldman writes
Functional nation states generally try to reduce the salience of socioethnic difference in favor of a national identity.
That's the crux of it. A successful group, a successful nation, is successful because non-members want to become members. Not like the UK leaving the EU, and not like Scotland leaving the UK.
I'm pretty sure the way to get people joining your group is to have the best economy.