I believe most people would want to be a "friend" rather than a "member", and I mean "member" in the technical sense of "guarantor member".
At this point, you're just guessing. The lists of people interested in being guarantor and supporting members were pretty similar in length.
If this is indeed the case, then once we have a "friend" level of membership (hopefully after the AGM, subject to it being approved), most people will sign up for that, which sorts out your problem.
I disagree fundamentally with limiting the number of (guarantor) members to 100, or indeed having any maximum number, simply because I believe that the charity should be as open to all as possible. What would people say if en.wp were restricted to 100 admins, and everyone else had to be editors?
Mike