Hi Tim and Barbara,
I had the same issue and if you want user auto-enrol it is a solution.
[If you have cohorts set up, then you can restrict self enrolment to particular cohorts by selecting the name of the cohort from the drop down box, A member of the cohort can self-enrol as normal; whereas a non-member gets a message telling them they are not able to self-enrol. - from: https://docs.moodle.org/25/en/Self_enrolment#Allowing_only_cohort_members_to_self-enrol ]
Did you find another solution?