My writing group founded itself. Many of us had been looking for a local group that suited our needs and not found one. The founding people met at a writing class, but were joined by others who'd never taken classes.
If you don't find an existing group, make flyers about a group's first meeting at a specific time and place (coffee house, bookstore, library meeting room--reserve space if required), the group's purpose, who's encouraged to join, and so on. Post them at libraries, new and used bookstores, coffee houses, diners, certain bars, and other places people who write might see them. Make sure you cover any college area nearby, as a lot of aspiring writers are students.
Be there--bring a friend if you like--and welcome anybody who shows up. Talk about what you each write, what your goals are, what you'd expect from a writing group, and see where it goes.
Maryn, whose group's been going for 15 years