Membership models can take several forms and don't necessarily need to involve money or salary. In fact, many repair groups run on a volunteer model. Membership models should address some key points:
Establish a point where people can qualify for membership. Regular attendance can be used as a criterion, where attending a certain number of events qualifies people for membership. Sponsorship can also be used to bring in members, where current members can introduce and vouch for prospective members. Consider how to address common but infrequent situations, such as members who are under a certain age. Think about whether you want to see at least one parent or guardian accompanying that person or whether a signed waiver will be sufficient.
Establish and communicate expectations of members, especially with regards to anything money (like dues). Attendance guidelines should be established, since these repair groups rely on community to thrive and be successful.
Once members have unique assets like keys to buildings & rooms and passwords, document who was what. From there, people can create contingency plans to address.
Divide your labor and make it redundant. - At least two people should be able to handle day-to-day tasks and responsibilities, designating one person as "primary" with the others as backups. This is crucial for digital assets like Facebook pages and Twitter feeds.