Visit http://www.planning.org/joinapa to register with the American Planning Association.
When you register with the national organization, you will have the opportunity to join a local chapter. Note that your primary chapter is the one that corresponds to your preferred mailing address. You may join as many optional extra chapters as you want.
The National Capital Area Chapter of the American Planning Association (NCAC-APA) listserve is a great way to get involved with the chapter. NCAC-APA offers our members a once to twice monthly e-mail with a list with announcements of interest to planners in the Washington, D.C. metro region. You will automatically receive email updates as an active chapter member.