If your students are on a set schedule - for example, one attends only on Mondays and the other one on Tuesday and Thursdays - you can assign this schedule in a single operation so that parents (and students, if they are using the app) only see the sessions that apply to them and can RSVP accordingly.
The Arbitrary Enrollment feature is only available for recurring series that repeat every week. See example below:
The Arbitrary enrollment then appears as an option in the main activity setting menu:
You can now set up arbitrary enrollment by clicking on "Setup Student Enrollment" in Activity Settings in Keedgo mobile app or web app:
You can now conveniently assign individual schedules in a single operation: