So far is seems the easiest way to add customers to a service is to go to the customer screen, open the customer, and add them to the service.
The other is to do it through the service and add customers through the search box.
Is there an easier way? I only have 133 customers to add but the current options I see are a bit tedious.
You can upload a CSV file, this support article explains how: https://support.logmeininc.com/gotoassist-service-desk/help/manage-customers-g2asd020005.
From the document you linked:
If I'm understanding this correctly, uploading a CSV with the intent of updating existing customers to add them to a new service doesn't work as the existing customers will be skipped. I'm not certain if updating via CSV is a solution. Please correct me if I'm wrong.