Could somebody please suggest some tags or terms to use for this sort of scenario. It could help me with search terms. I am guessing at some of the tags here...

I think the tag we want for this is multi-tenant - https://en.wikipedia.org/wiki/Multitenancy . Though, happy to make a more "well known" tag, if we can figure it

I manage a Cloudron with, wait let me count... 50 domains.
Each Domain has it's own Website and E-Mail I/O.
~150 Users

Each Domain has its Cloudron Group which can access only their Domain Website and apps.

So yea it works.

This setup also triggered this error
https://forum.cloudron.io/topic/5474/installing-webmail-app-leading-to-database-error-er_data_too_long-data-too-long-for-column-value-at-row-8

What I generally do is to create domain-based group https://docs.cloudron.io/user-management/#groups e.g. catforum.com and then only make that instance available to that group.

You also likely want to make sure you don't have any apps that are "Available to all users on this Cloudron" otherwise everyone will see those.

And of course the my.cloudron.com domain would be the same for all users of all forums, but in many cases (e.g. if it's only the forum and not other things too for the same community) you needn't even tell the forum users about that and instead just direct them to their specific domain. But if you have limited everything by domain-based groups then even if they do login to my.cloudron.com they'll only see the apps they have permission for.