Is your group a member of, or related to, a larger umbrella organization? For example a church group might contact the denomination's main office where they may already have a link with a travel agent used to dealing with such group travel and its unique demands. They may also know how to obtain free luggage checking too, which is a boon if you are carrying in supplies.

Last week on a trip to Florida, the two surgeons next to me were headed for Haiti and told me that they had checked 7 bags filled with medical supplies, and US Air had not charged them any baggage fees. That astounded me, given that US has been known to charge passengers for water in-flight. However, they were traveling to work with a large, well-known charity which may have "encouraged" such generosity.

You may want to investigate a charter flight for your group also - you will likely have more flexibility.