SQL Server Reporting Services security is managed on two levels: the SSRS site and on items within the site. At each level, the tasks that users can perform are managed via roles. Roles are just groups with certain tasks assigned to them and members of the role can perform the assigned tasks. It's that simple. Taking some time to create appropriate roles and assigning users accordingly will ensure that your SSRS site, and the reports it houses, are secure.

-My advice for you is create Two Folders "A" and "B"

-Create custom Role for example "Folder_A_users" and add that user to this role from Site Settings

-All other reports add them to Folder B