Groups have a site_id that is barely used

[prioux]

I think it's only used to establish a connection between a SiteGroup and its Site. We may want a better mechanism. Maybe the same link as between a User and UserGroup (by name).

Otherwise, it's misleading, as any new workgroup a user create gets a site_id set to the user's own site, and they stay like that even if the user's site affiliation change.