schema design question

We're trying to design a table to keep track of names and addresses for a membership group and also people who inquire about membership. Obviously, only members would have member numbers, but everyone should have their own serial numbers. The way it was done before was to issue each new record an informix serial number, and hand-issue the membership numbers when needed. Unfortunately (actually, it's fortunately), the number of members is increasing rapidly and we want to issue their numbers serially. I'm sure someone has solved this problem already. Anybody want to share?