@Photorat Looking into it a bit further, what I've found so far:

- Mobilizon groups can be followed from eg. Mastodon to get updates: https://docs.joinmobilizon.org/use/users/follow-federation/

- But you cannot participate in other ways from Mastodon: https://framacolibri.org/t/integration-with-mastodon/16446

- It does not seem specified whether you can use one Mobilizon account to fully participate on another Mobilizon instance (but the way the rest of the documentation is written, it seems to imply you can't: https://docs.joinmobilizon.org/use/administration/federation/)

- You *can*, however, open a group for 'anonymous' participation, so that people can sign up for events without an account at all. This seems to be limited to registering your participation in an event, however.

So I think that if you want everyone fully 'on board', and able to fully participate in the group, they would probably all need an account on the specific instance that the group exists on.