How do I integrate Doorkeeper gem with a Multitenant SaaS Application?

So I have a Multitenant SaaS Application where users signup, pay, create an account and start using the service. Now I have received a requirement where this service needs to have a andriod client which needs to be global to the application and not the tenant. Now how do I make my SaaS application to act as a OAuth2 provider and authenticate and authorize the client?

If the user wants to sign in via the Android Client, at that moment I do not know which tenant/account the user belongs to so I cannot confirm the subdomain in play for confirmation.

Can anybody suggest a good way to handle this?