The ID token, which is always guaranteed to be in the JWT format, will contain a sub claim that represents the user identifier in a unique way (within the realm of the authentication service associated with the token).
This means that after validation of the ID token you can extract a unique user identifier from within it; this identifier may not be a 32 digit GUID, but it will be unique so if you really need a GUID you can just generate a new one and associated it with the identifier contained in the sub claim in your own database.