All you have to do is enroll one sim. After that, choose a random destination (doesn’t have to be at the university) then travel with the sims you want to enroll.
Once there, activate cheats with “testingcheats true” then shift click each sim you desire to enroll And click “university debug” there will be an option to enroll selected sim. Boom.
If you want to enroll a sim that is not in your family then simply travel with them, turn on testing cheats and shift click and “add them to family” prior to the university debug. You’ll be done in less than a minute!!