IMHO, the only fair way to do this is:

(1) Plan for the largest group possible.
(2) Establish a firm final date for registration.
(3) Accept registrations on a first come, first served basis *unless and until* the number established in (1) is exceeded.
(4) If/ when the limit is exceeded (is there really any question that it will be?), have a lottery with family member clause (don't want to seperate husbands and wives and children, do we?) to decide who goes.
(5) Among the lottery winners, those who registered first should get the lowest number reunion pieces, or that could be by lottery, too.

-Art