Congratulations! I know exactly how you are feeling.
I went to our local travel agent and got a group contract with Air Transat. I recommend you to do that early, as the pricing will likely be better. This locks you into a group rate. The agent will give you the deadlines for deposits, and what I did was created a wedding website to stay in contact with all the guests. Our agent took care of all the travel arrangements for our guests.
If you have any questions, don't hesitate to ask!
Jennifer