To manage users separated by site in Sitefinity, it is necessary to set up a Membership Provider for each site. Each website must be configured with the corresponding Membership Provider using this setting. The setting maintenance displays the membership providers that exist in Sitefinity. 

After creating the provider,  you must restart the application by modifying the Sitefinity web.config file or following the steps in the Sitefinity Documentation (https://www.progress.com/documentation/sitefinity-cms/administration-application-restart).

After the application restarts, you will see that the new provider is active.

Beginning with version 7.1 standard Sitefinity Site-Specific Users will be utilized to manage Membership Provider.  For more information click here.