This will dynamically apply the role permissions for sites a user has been assigned to.  This is intended as Multi-Site environment feature only.

Step 1: Create or edit an existing role to be defined as a Dynamic Role that will assign permissions to users based on the sites they have been give access to.

Leave sites unchecked as Site assignment will occur when users are created and given a site assignment.

Step 2: Create/Import your users and assign them your dynamically created role.  

Step 3: Make sure users have been assigned to a site within the Manage User setup.