1. Create an Employee Profile (HR Tools > Employees > Profiles) and tick only the permissions they need, e.g. List Employees Schedule and Sign Up for Shift Requests.
2. Open their record under HR Tools > Employees > Employees and choose that profile in Permissions Role, then Save.

The person signs in with their normal user login and only sees the menu items their profile allows. Choose No Admin rights later to revoke access while keeping them as an employee for clock-in and scheduling.