1 Free for every 10 registered.  Pay for 10 and bring 11.  Pay for 20 and bring 22.  You get the picture.

Register at least 10 at the going rate and you can add more to your number later at that same rate!  Possibly saving your group hundreds of dollars.  Example - Register 10 at the early bird rate of $20 per person and you can add more to that number later at the same cost even when registration cost have increased.