Meeting spot is Peirce Mill in Rock Creek Park now in the south (larger) parking lot. Check forum link for weekly details (tweaking the meet-up times/distances/etc.).
Map 1: Click for Meeting Spot @ Peirce Mill (general intersection pictured below. Actual meeting spot is on the south side of Tilden, west of the creek in the south parking lot with the bathrooms.)
Map 2: Click for a map of the often run 9-10mi route (lots of folks run shorter and vary the loop with so many options on the trails.)