1. Check this topic on How to access buddypress membership restrictions settings ?
2. Select Restrictions Type :
- General Restrictions : Means the settings for the "Global" User Role will be applied on all the members ( logged-in users ) except admins.
- Roles Restrictions : Means you can set a different restrictions settings for each role available on your site.
3. Now depending on which type of restrictions you want to add, check this topic below :
- How to apply global restrictions on all site members ?
- How to apply restrictions by user role ?
- How to set visitors restrictions ?
4. Check this topic below on How to customize notices that are generated when the user tries to perform an action that is not allowed ?
How to set custom restrictions messages ?