We have implemented a multitenancy app with the Acts As Tenant gem (https://github.com/ErwinM/acts_as_tenant) and Devise, where a User can have Memberships in multiple Organizations. Users eligible for managing other Users (you my call them Admins) can 'invite' Users from other Organizations to join, so that they can switch between those Organizations. No subdomains used.
Essentially a User
has_many Memberships (and Organizations
through Memberships). An Organization
has_many Memberships (and Users
through Memberships). And a Membership
belongs_to an Organization and
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