You can book directly with the resort if you choose, but you will have to be extremely careful when trying to negotiate group bookings directly! If you do not read the contracts very carefully sometimes you can be on the hook for money in the event that guests start cancelling or booking through another source. It can become complicated and very stressful if things aren't handled well.
My recommendation is to use a travel agent. They will be able to help with a lot more than just the travel arrangements and you know that you will be safe when it comes to contracts and negotiating group rates. You can use an agency that deals specifically with wedding groups and this way your guests will have the option to book through them too. You may not get a "discount" per say, but often travel agents are able to negotiate really reasonable group rates. Also it sometimes happens that if you book enough people under your groups you could quailfy for promotions such as free seats or rooms and then you could use this to disburse a discount to your entire group, or use it for yourself.
Travel professionals will have insite that you cannot be expected to have and should be able to provide guidance and peace of mind!
Blue Petal Destination Weddings