Nature Centers in the DC area
Primarily educational:
Audubon Naturalist Society
Mycological Association of Washington
Maryland Native Plant Society
Hiking and outing clubs:
Capital Hiking Club
Center Hiking Club
Maryland Outdoor Club
Northern Virginia Hiking Club
Potomac Appalachian Trail Club
Sierra Club Potomac Regional Outings
Wanderbirds
Washington Women Outdoors
There are also many outdoorsy Meetups in the DC area.