On the employee page, under Roles:

1. Click + Add Role.
2. Pick a Job Role (or "No assigned role") and type the Hourly Rate (e.g. 18.50).
3. Add more rows for each additional role; use the row's remove button to drop one.
4. Click Save.

Each role can appear only once per employee (duplicate rows are ignored). The rate is used to calculate Estimated Wage on timesheets: when a timecard is created for a role the employee has, that role's rate is used; if the shift's role is not in the employee's list, the rate of the first role on the employee is used as a fallback. An employee with no roles at all cannot clock in and cannot have timecards.