As HOWMAR suggested, assuming you can get groups of 10 or more to fly on the same flight, contact the airlines for group rates. I would stick with either American or Continental.
For the people coming from Tampa, maybe American connecting through MIA or SJU. For those coming from NY/NJ, Continental has direct flights from EWR. (You didn't mention what airport in the DR you will be flying into).
Here's a link for Continental. It looks like they will be able to give you an online quote.
In answer to one of your questions, unless you are able to get a fantastic bargain, I wouldn't book now. You didn't mention exact (or even approximate) dates, so I can't give you an estimate of how much to expect to pay. In general, the closer in December your travel dates are to the holidays, the more you can expect to pay.
If this is all overwhelming to you, use a travel agent. A good one should be able to get you the best rates; especially if you also need hotel rooms.