When you have multiple blogs the admin of blog_10 can easily search on username or emailaddresses to add a subscriber of blog_3 as an admin or with other privileges to his blog.
Is there a way to disable this and let a administrator only search on users that signed up for his own blog so he only can add them with extra privileges to his blog ?
I would have users that signed up sticked to a specific blog and not as subscribers at all on other blogs too.