If you need a subsite with access that is restricted and separate from the rest of a SharePoint site collection, follow the below steps.
Note: Full control access or higher is needed. If you don't have the right access, contact the site owner/admin or EASI SharePoint Support for assistance.
Any permission changes you make in this area will only impact the subsite and nothing outside it.
To re-inherit the top site level permissions, select Delete unique permissions > OK in the Advanced permission settings.
To remove groups/users from the subsite, select the box beside their name(s) and select Remove User Permissions (at the top).
To add groups/users to the subsite, select Grant Permissions (top left). In the pop-up, type out and select the name(s). Select Show Options to assign the level of access and if they should receive a notification. Select Share.