I needed a summary at the end of each page. I found nothing better than giving each record a page number and grouping by page number.
In your case, I would go the long way: Count the number of lines of each group to calculate the position of the summary and print out the report peace by peace. Ex: Print A B (not C as it is on 2 pages, then C D E (not F as ...), ... I would probably use in this case a specific Group table to set the calculations and steer the reporting.
Solution 2: use a specific page field and give each record its page, as a second summary field. In v6 there is probably the possibility to put a page break on a summary.