American Association of Community Theatre. “AACT is the national community theatre organization, whose chief goal is to enable community theatres across the country to become the cornerstones of the creative life of their communities—to provide quality entertainment, intellectual stimulation, challenge and opportunity, and to be worthy contributors to an improved quality of life for the communities which they serve.” (https://aact.org/aacts-mission-purpose 2019 March 5.)
CORST
Council of Resident Stock Theatres. Little information is available online. These theatres maintain contracts with Equity and SDC. Signatories of the SDC CORST contract currently include: Barnstormers, Flat Rock Playhouse, Maine State Music Theatre, Mountain Playhouse, Peterborough Players, and Williamstown Theatre Festival. (http://sdcweb.org/contracts/agreements/corst/ 2018 March 3.)
An organization of more than 200 theatres, companies, and performance spaces in the Chicagoland area.
League of Resident Theatres. LORT is comprised of 75 theatres in 30 states plus DC. They maintain contracts with Equity, SDC, and USA, but sometimes have contracts for non-union performers and artists, especially during the summer. LORT theatres provide excellent experience for early-career professionals by giving them the opportunities to work with people who know the business and have "made it".
If you check out the Work page, and you find broken links, go to the theatre's home page and try the "About Us", "Contact", or "Events" pages. They may also be listed on the Summer Theatres page on this site.
National Theatre Conference. "Founded in 1925, The National Theatre Conference is a not-for-profit organization made up of distinguished members of the American Theatre Community." (http://www.nationaltheatreconference.org/ 2018 March 3)
Theatre Communications Group. TCG is a national theatre organization for professionals, educators, and students. They work to keep communication between members, supporting networking and education, as well as providing grants, research, and advocacy of various topics of concern to the greater theatre community. TCG publishes American Theatre Magazine and ArtSearch.