It sounds like you need to create a subsummary part and sort by Customer Site.
That way you get something like this:
Customer Site 1
Customer Site 2
Customer Site 3
If you already have a sort you can just add the Customer Site field to the sort. Just pay attention to what yo sort first and what you sort second.
The ustomer site would most likely be a Merge field in the Sub Summary part.
The product portal would go in the Body part.