I'm sure there's a simpler way, this is just off the top of my head, but you could use a Summary Criteria that uses a CASE statement in a Formula, where if the item's process group matches what you're looking for, then 1 else 0 end,
If the Sum of that formula is greater than 0, then the sales order contains at least 1 item with that process group (I don't know what process group is - I'm assuming its a List/Record field?).
And if you don't have Main Line filtered to Yes, then it should show all the lines of each Sales Order that has at least one such line item.
Let me know if that works, or if you found a better way.
On second thought, that would probably evaluate for all sales orders in the results set, not just by one sales order at a time.