I accidentally started the Tucson Audiophiles back in 2013 when I hosted a few informal demos of the wildly popular NCore amp and compared it to a Van Alstine Synergy 450. The first attendees were a mix of friends, word of mouth and AudioCircle members. We started gathering at other homes and then the Tucson Audiophiles Google Groups was formed so we could keep in touch.
To start a group you will need a reason for people to make room in their busy schedule for you, since at first you will be in charge. Later on that will change and the group will evolve/devolve in directions you never imagined. Luckily our group has evolved into a terrific group of friends. We loan equipment to each other, listen to new music, trade CD's and have casual comparisons, nothing beats trying out new electronics or cables in your own system and having a half dozen friends commenting. Every few months it is like a mini-RMAF without the cost.
Free Bar-B-Q, beer and whiskey is very helpful in starting a group too.