New User Registration From Subsite

Hello,

I am seeking a variation new user registration scenario for subsites on a multisite installation.

I want users to be able to land on a subsite in a multisite network and register there as a user only (no blog creation) without being redirected to the main site.

Several plugins I have tried can keep a user on the subsite they land on for log in, but things fall apart if a user attempts to REGISTER on the subsite when they already have credentials on another site on the network that they may have no knowledge the subsite they are on belongs to.

Even after overriding the normal re-direct behavior of redirecting all new users to the main site and keeping them on a new user registration form on the subsite, if the user attempts to use the same email that they have used to register on any other site in the network or the main site, then they encounter an error and are confused since they have never signed up for this subsite, but the message tells them their email is already in use, etc.

Because the user may have no knowledge that another site they joined belongs to the same multisite network, new user registration confusion is understandable.

How are others handling new user registration on a subsite when the user already has other credentials on another site on the multisite network that they do not know they are part of?

When a new user attempts to register and the user registration form searches for the existence of the email they enter perhaps is just could just use the email to add the user to that site without trying to add it to the database again?

Logic:
If new user registration email is already in the database then perform add user to this site only. Do not attempt to add to database again.

OR something more logical and friendly....

Thanks,
Phil D