We have had the exact same problem as you! We booked through Sunquest also. We have now lost all of our seats at our group rate because the flight is "sold out" and people now have to pay $400 more now to get a seat. We are still 5 months away from our wedding so we were very upset about this. We've had 43 people book already, but have several more that still want to come, who may not now due to the increased cost. When we signed up we put a deposit down to hold onto 10 seats also, but the extra that weren't sold were taken back from us just last week so now we aren't holding onto any spots for additional guests. You'd think since we've given them so much business already that they'd pull some strings for us, but apparently not.
From what I understand, this is how Sunquest works:
-Sunquest charters fly with WestJet airlines
-When WestJet adds a flight, they sell a large "block" of seats to Sunquest before making them available to the public (for example 40% of the plane)
-Sunquest gets these seats at very large discount, and they can then sell them to the public at the great discount group rates
-Once all of Sunquest's alloted seats are used up, they need to put in a formal request to WestJet for more seats on the plane if a group needs more seats
-WestJet charges Sunquest a fee per seat, which is then passed along to us the consumers (eg, the extra $50-400 you were told about)
The thing to keep in mind, is that the plane is not actually "sold-out", only Sunquest's portion. WestJet flies to Jamaica several times a day, so your travel agent should be able to get people there on the flight or another one. And also, it's likely that the trip will still be cheaper using your group rate and the additional fee than paying for the entire trip separately.
Apparently from what I've heard, next year is one of the busiest destination wedding years they've seen in quite a while, which is why seats are selling out so quickly.
Unfortunately there's not much you can do except for encourage people to reserve their spot if they want to come. I've been to destination weddings before and talked to other people and nobody has ever had this problem in past years. They've all had people book last minute and still get the group rate.
It is totally unfair, we weren't told this may happen and were given no notice, just all of a sudden the group rate was gone. But there's nothing that we can do about it now....
So to any new brides out there - If you are booking with Sunquest, get your guests to pay their deposits ASAP!!! This could happen to you too!!!