By default
Owners, managers and receptionists can view client records. Providers see the customers on their own appointments, not the full client list.
Granting access to one person
An owner can grant client access to an individual team member from that person's profile under Team. The change applies to that person only.
Enforced everywhere
Permissions are checked on the server, not just hidden in the interface, so the Clients screens, exports and searches all respect them.