If you are a professional agent registered with HMRC and have an agents Gateway ID and password then you can use this to file all your clients tax returns. You can enter these details in settings when logged in as the 'master' user.

If you are completing tax returns for your spouse/partner or other friends and family then you will need the Gateway ID and password for each person that you are filing for. If they do not have a Gateway ID and password then they will need to apply for one via the HMRC website.