We ask people to tell us if they are coming, but we do not require reservations-everyone who shows up is fed. We try to estimate attendance in advance by requests for crash space plus talking to local people. In addition, we try to make our feast plans sufficiently flexible so that we can scale the feast up or down at the last minute. I do not think it is practical to make all SCA events free. Some are so large that they would bankrupt even a very wealthy group.
Some groups have no sources of income adequate to pay the cost of even a fairly modest free event.