We had a few guest at first say they weren't going to book with the group and wait for a last minute deal. This made me really stressed that they wouldn't find a flight or the hotel would be booked or something, plus its tough to plan a wedding when you don't know concrete numbers. That being said, we encouraged our guests who wanted to book last minute to call and talk to our travel agent. I don't know exactly what she said, but they all ended up booking with our group rate.
Our group rate is closed, but we are still planning on having a few straglers book last minute, therefore, we are kind of using just an estimate at this point. Mabe you could try to explain to your guests, or have your travel agent explain, the reasoning behind group bookings and why they should book. Good luck with everything!