Where to meet up in San Francisco

People sitting on the steps of Union Square's plaza in San Francisco, with the Dewey Monument column and department stores around it — Union Square, a few blocks from the Powell St station. Photo by Bernard Spragg (CC0) via Wikimedia Commons

In San Francisco, a group coming from the East Bay, the Peninsula and the city's own neighborhoods usually meets along Market Street. Four downtown stations there are shared by BART and Muni Metro, so most people can reach them on one train.

These are the areas groups pick most often:

Union Square. The Powell St station has BART and the J, K, L, M and N Muni Metro lines, and it connects to the Powell-Hyde and Powell-Mason cable cars. The T Third stops at Union Square/Market Street.
The Financial District and the Ferry Building. Montgomery St and Embarcadero have BART and the same five Muni Metro lines. Embarcadero is close to the Ferry Building.
Civic Center. The Civic Center/UN Plaza station has BART and Muni Metro, near City Hall, the Main Library and the Asian Art Museum.
The Mission. The 16th St Mission station is on BART, with four routes.
SoMa near Oracle Park. Caltrain from the Peninsula stops at 4th and King, where the N Judah and T Third also stop.

Getting around

BART runs from 5 a.m. to midnight on weekdays, 6 a.m. to midnight on Saturdays and 8 a.m. to midnight on Sundays, as its schedules page says. It doesn't run overnight.

From 12 a.m. to 5 a.m., Muni's Owl buses run every half hour. The L Owl and N Owl buses replace those two Muni Metro lines at night.
Downtown, Muni Metro trains stop on the level above BART in the same four stations: Embarcadero, Montgomery, Powell and Civic Center.
The T Third runs in the Central Subway under Stockton and 4th streets. Its Union Square/Market Street station connects to Powell.