TravellerPlus I don't think they're trying to do everything on a single booking, but rather looking for a tool to help narrow down destination options based on the various departure locations.
I second the use of Google Flights (or something like Kayak or maybe SkyScanner), mentioned above, where you can put in your departure and then choose "anywhere," then you can filter down from there.
Another option is to see if the airline web site has a simple list of destinations. Southwest has this, where you can just see everywhere they fly. If you all are originating in cities with a Southwest presence, that would be a start.