I Would not fly on a charter to save my dog's life
(Though he died some 7 years ago).
Service is pathetic, never on time, add stops where and when you were not told in advance and have 1 inch of leg room.
And they fly weekly whereas I fly sometimes 4 days sometimes 9 days, etc.
Find the tour catalogs in a travel agent that have DR as a destination - go to the web sites of the tour companies (e.g. in Canada Sunflight, Conquest, AlbaTours etc.) and you will find contact info for the tour operator and often a company they own that also sells the packages and then deal direct.
Otherwise for 100 people you might get a convention fare from AA or Continental, they gave group sales offices and you can ask them.