[quote=@Bosco]
Usually people band together for a common goal, and while fighting crime and saving people is good reason any hero can do that on their own. One reason to bring heroes together is that there is something too big for just one to take on like some sort of massive monster or a evil organization. Another way we could do this is that they are invited to do so by a hero with a lot of pull and popularity.
[/quote]
Or fighting off sometime of invasion/take over attempt of the city...