SAGE can also provide you with a list of SAGE members in your area, as long as you agree that you will only use the list for the purposes of forming a non-profit group to benefit system administrators. Once you have a core group of people, you'll want to set up a mailing list for them, and then meet in person. The first three have to be done almost immediately, while final decisions about structure can usually be put off for several months. First, you'll need to find a space for the larger group to meet in. Your best bets are the employers of the core group.
Most groups meet in the same place on the same day every month.
This works, and it makes it easy to avoid re-using a meeting space that turns out not to work after all, but it requires a lot more work for both the organizers and the membership at large. The requirements for doing this vary widely depending on your location and your meeting plans, so each case needs to be discussed separately, but you can assume that doing this will require the guaranteed presence of at least one current SAGE member at each meeting. Most groups meet in the evening, on weeknights, on the theory that people's employers won't like them using work time every month, and their families will want to do things with them on weekends. They tend to specify times like the first Wednesday of every month or the third Thursday. What day you pick will depend on what other groups schedule events in your area and when they schedule them, and probably on trivia like the schedules of the meeting organizer and the first building you find.