"for an accumulation" is referring to the summation of tens of thousands of cyclists from the various bike events, not just Vancouver's...which leads to support for human-powered transportation, and safer streets and at these events, the participants participate nude - it is hard not to feel positive about your body if you are willing to cycle nude in public, so I guess if you will participate in this, you will very quickly learn to be positive about your body!

Its saying that the event is taking place at the same time as other events so there will be more people in the area to participate. Body positivity is thinking of one's body in a positive aspect instead of a negative one. If you don't like part of your body the event will try to promote a positive feeling of those parts people don't like.