how would I set the primary part for a group of models?

Assuming a group of identical models have the same relative primary part, how can I set all the primary parts at once in a script?

The primary part has to be inside the group, so you can't have different groups with only one primary part. What you can do is group all groups together with the main part and set it as primary.

Like this:

• Group
  • PrimaryPart
  • SubGroup
  • SubGroup ...