If your employee signed up to the Request Mobile App without an invitation, they can add you as their employer (→ they can do that by following these steps). Once added, you'll be notified via email and asked to confirm them as an employee of your company.

To confirm them, navigate to Payroll & Expenses > Employees > "Pending Requests" and hover on them to click "Approve" or "Reject".

Once approved, your employees can submit expenses via the Mobile App, and you can reimburse them.