Once Single Sign On (SSO) has been established for a domain, users must have an email address that matches that domain to access the tenant.
Once a domain is linked to a tenant, users with that domain cannot be added to other organizations’ tenants.
Refer to Syniti Knowledge Platform Single Sign-On (SSO) and Domain Access in the online help for an overview of the SSO implementation within the Syniti Knowledge Platform specifically in regards to users that have credentials in more than one domain.