For a producer group with many producer group members, do I have to add the sites and products one by one, or will we have the possibility to use an upload sheet?

Adding site(s), PHU(s), product(s) and linking the product(s) to their respective site(s)/PHU(s), the bulk upload sheet can be used.

The upload sheet is stored in CB-AT Portal.

Here is the direct link to the Bulk Upload Tool.