Since I get this in the ask over here pretty often as well, I’m just going to copypaste this lovely message from our fearless leader Hazard over on the FB page. He says it all much better than I could, and he definitely has more knowledge about it, since he pretty much built our group out of nothing.
We get a lot of questions about how to start your own Umbrella group. The best answer I can give you is this. Create a FB page like this one and stay active with it. Make sure your page name tells what area you are from. Be active in your area and go to conventions and other public events where people will recognise your costume. Make sure of course that the event is appropriate. Do not be afraid to talk to people, networking and making friends is what will make your group grow. But most of all be professional and be active. Here is a picture of just me at one of the first small event I attended by myself. Hard work really does pay off. ~Hazard