I did once have it set up like this but it took me along time to find out how to do it. I had it set up so that a user registering on one site would only become a subscriber to that site and it worked a treat.
Unfortunately, something went wrong and I had to deactivate the plugin and, since then, I have not been able to work out how to do this.
This is a brilliant plugin and is well advertised throughout the WordPress world but if only someone would answer the support questions, it would be even better.
There are a lot of people asking this very same question, so this is an appeal to the owner of this plugin – please answer this question.
Regards,
David