I agree about SAM/IBM.
a small group of magicians in my city started a group awhile back called “The Monday Night Jams.” We now have over 1,000 members worldwide in 6 major cities!
The way we did it was simple. We got 5-6 people together and found a place to meet. Each of us told our friends and they told theirs etc.
now we had it easy because we already have a bunch of clubs out here (castle, ibm, magic shops, etc.) But the beauty of the internet is you can post on forums or classified ads on Craigslist or on Facebook. Get the word out and get a few people involved. Those few people will bring friends and it will begin to expand.