Here's a guide for Mastodon admins on how to adjust who can sign up on your server:
🌱 https://fedi.tips/controlling-sign-ups-on-your-mastodon-server
It's not just the registration settings, there are also invitation settings to adjust, and these can be combined to give a range of different sign-up policies.
The guide is new, hopefully it's okay. Any feedback/corrections welcome.
