Sub claim/field in id_token & userinfo endpoint

From the documentation and examples, I read that sub value has to be unique that does not change.
We are planning on building multi-tenant solution, which authenticates users across different organizations using single OAuth server; And so they may possibly have same user name in different organization as well. Of course the client id being used across these organizations would be different though. In that case, is it ok to use the same user name as sub field or does that have to be made unique by appending organization identifier like [email protected] or so?
Is there any offering from Hydra to support multi-tenancy?

Subject fields MUST be empty. How you achieve that uniqueness is up to you.