At Comair we had about a dozen pilots who were on property before the union got voted in. They did not join and they did not have to pay any fees. Comair is an agency shop. So that is why I don't think you have to sign up. I believe it is a one time decision.
I am aware of this situation at Comair. I am guessing that this arragement was made between the Union and the company in the negotiations of their first agency shop agreement. The Railway Labor Act (which governs multi-state common carriers by air, as well), does not have any so-called "right-to-work" provisions as many state laws do. The federal RLA preempts any state laws to the contrary and allows the parties (company and Union) to negotiate their individual provisions. Most that I am aware of agree on an "agency shop," as NetJets/NJASAP have done, and as Comair/ALPA have done. In that process, they may elect certain exclusions, as Comair did, but it is not a certainty.
One of the reasons that RLA Unions prefer agency shops to open shops is that we are legally required to provide "fair representation" to all members of the "class and craft" we represent. Because there is a cost to that, even if an idividual elects to not become a member, we have a duty to represent his interests, which may include costly arbitrations in some cases. Hence, it seem fair to charge some fee for that (fee payers, by law, are only required to pay for services related to representation and contract administration).
NJ/NJASAP pilots have the option to be fee payers, rather than Union members. Obviously, when NJI pilots become members of the "craft and class" they will have the same options. Those who make that election will be represented by NJASAP, but will not enjoy the benefits of membership (ie, cannot hold Union office).
Hope this helps,
Brian