When you invite a user to your tenant using an externally provided account, they become a guest account in your tenant, they will connect into the portal and likely go to their home tenant. They will need to change directory to do to your tenant. He won't need to use any other credentials, but access in your tenant will be limited to which resources you have granted the guest account access to.
To identify b2b users, generally, you will go to AAD and filter in member type = Guest (unless you have converted accounts to member), in which case you can search on those which have an 'invitation accepted' status of yes.