How is this accomplished? There are no leaders. The route evolves organically. The front of the pack stops for the lights at the intersections, but to get the group through the intersection together, some more experienced riders act as a cork to hold up the cars for a little while, if the light changes before every cyclist is through. Keeping the whole group of cyclists together is a safety net for all, as individual cyclists are very vulnerable and even small groups experience more dangerous situations when forced to mix it up with the automobile.
Drivers in Toronto are notorious for parking in cycling lanes, veering into cycling lanes to turn right at intersections, not "seeing" cyclists. For cyclists, "being doored" is a nasty event one watches for whenever passing parked cars. Proper signals and lane-changes not withstanding, a cyclists had better actually look an oncoming driver in the eye and get a wave to ensure safe passage!
As a runner, I have discovered that if you are not in a large motorized vehicle, you are invisible and therefore liable to be run over. Or if you are visible, you are yelled at, cursed at, and told to get off the f------ road!
About an hour into the ride the police arrived to "organize" things. As the police directed traffic and enforced the traffic laws, the peaceful, happy mood began to evaporate. The mass became fragmented and the situation became uncomfortable again as bicycles are really no match for cars, SUV's and trucks.
The police started targeting cyclists they perceived to be the "trouble makers", ticketing them for traffic violations.
Eventually, the groups were so fragmented that it was no longer the safe, happy ride through downtown Toronto, but the usual grim battle to negotiate through traffic that either doesn't see you or that believes you have no right to be on the road.
The fun went out of it for me by then and all I wanted was to get home safely.