San Francisco's critical mass was successful to the point it almost doesn't exist anymore (it's not necessary anymore with better political engagement). I believe it was successful because it:

• started at the same location and time (once a month on a Friday)
• occurred during evening commute hours
• had no formal leadership
• no planned route until just before departure

This combination meant authorities had no ability to shut it down. What office could the raid? How could they bring a lawsuit?

Once a month on a Friday meant it didn't have to be about your commute. Rather you'd leave work and ride to the start point. It was a protest first and a utility second (though the route did start from the financial center of the city).

Take a look at bike parties if you're looking more for a community ride. They bring more of a general supportive base than as a protest.