Option 1: Less work but less flexible
Select your portal while in layout mode and use Format | Set Sliding/Printing... make the portal "slide up, also shrink enclosing part". This works as long as you don't have more portal records than your portal can display. Also, the sliding is not visible in Browse mode, you have to preview, PDF, or print the report to see the sliding.
Option 2: More work, but much more flexible.
Create a list view summary report based on your portal's table. Place fields from the parent table in the header or in sub-summary parts.
Be careful to sort your records in an order that includes the fields specified as "when sorted by" for each sub-summary part you create and the Sub-summary parts will not be visible in browse mode if you are using FMP 9 or earlier.