Opensource project Hitobito seems like the perfect system for managing members, mailings, registrations for Groups, Events and Courses for Hackathons, Meet-ups and Online courses conducted at Ungelich.ch and friends.
Project Website: https://hitobito.com
Github Repo: https://github.com/hitobito/hitobito
Project Demo: https://demo.hitobito.com/en/users/sign_in
User Guide (in German): https://hitobito.readthedocs.io/de/latest/