Waitlist for guests
For certain programs only allow the registration open for library card holders and add guests to a waitlist until a certain number before the event happens. At that time, move the guests automatically to the registration if there are still seats available.

When creating an event, library staff can set a date to open registration to guests. This allows their library patrons the opportunity to book programs first.
https://portal.productboard.com/iii/15-vega-product-portal/c/995-open-guest-registrations-on-a-specific-date