Library bulletin boards and newspaper ads are the most common. Sometimes they are featured by local columnists. University writing teachers often know of groups. University bulletin boards sometimes list them. Writer's groups are very frequently sponsored by community arts councils - contact your local library or county government.
Some cities have active film groups.
For example, the Kentucky Film Artist Coalition regularly sponsored meetings for writers and actors and sponsored seminars with major talents from New York and L.A. Denver and other cities have chapters of Women In Film. If you find nothing in your community, try starting one yourself - meeting at home is common, acceptable, and often preferred. Or ask the local library or government if there is free space available for a weekly or monthly meeting. Advertise for members in the newspaper, library bulletin boards, local supermarket bulletin boards, etc. One of the more fun things about screenplays is that they can be worked with by actors.