User accounts can be shared among all sites running on one Kentico CMS installation. This means that if a user creates an account on one site, they can automatically log-on to the other sites running on the same installation using the same credentials.
This behaviour can be switched on or off in Site Manager -> Settings -> Membership, using the Shared user accounts check-box.
|•||If the check-box is enabled, user accounts created on one site will be shared among all the sites running on the installation.|
|•||If the check-box is disabled, new accounts will be assigned only to the current site and not the others.|
However, registration web parts have the Assign to sites property. Using this property, you may determine which sites the user accounts created via the web part will be assigned to.