Show/hide forums based on the domain? (Shared forum via CNAME)

[Falco]

@tommy87

I’d be willing to put in the config time if I had some place to check out some documentation

Since this is an unsupported and unusual configuration there is no documentation.

@tommy87

The second option may work as well if I can find a way to automatically put someone in group membership based a modified Sign Up link we provide on each specific site maybe?

This is the way to go and our SSO protocol comes with support for it out of the box. You can pass the group membership in the SSO payload. Documentation is DiscourseConnect - Official Single-Sign-On for Discourse (sso)