Finding at least one local non-profit partner is essential to pulling off a good medium to large event. The main reason is simply because these organizations have established relationships with the people you seek to take portraits of and they're in regular communication with them. This is a huge benefit when it comes to promoting your event and ensuring that people show up for it.
Here’s a list of where we've been most “successful” in serving large groups of people:
Rescue mission
Womens health
Halfway houses
Recovery programs < this is very unique experience because so many of them deal with poor self-image issues
Food kitchens
Churches
Homeless organizations/ministries
Check out this blog post for more details about securing local non-profit partners.