This approach uses container to container communication. This requires that the bug in /overleaf/services/web/app/src/Features/User/UserRegistrationHandler.js is fixed. see https://github.com/overleaf/overleaf/issues/1206
We can block users via the tools/blocked_users.json. And we allow domains for tools/allowed_domains.json for which creation of account is allowed. e.g. uni-bremen.de allows @*.uni-bremen.de and @uni-bremen.de. Furthmore, invited people can also create accounts.
# No google
I replaced the google captcha because of data privacy reasons... Just, kidding I wasn't able to make it run. Thus I replaced by a python solution.