Hi Hayley24,
Yes, if an employee already has other employment income or a pension, you must register for PAYE, even if their pay is below the £123 weekly limit.
The employee should fill out a starter checklist to declare their situation.
Typically, they will choose statement C, indicating another job, which means you need to deduct tax on a BR cumulative basis.
You can register as an employer here:
Register as an employer and the link to the starter checklist:
Starter checklist for PAYE
Thank you.