Excellent question. I added portfolio groups in response to a couple of requests from users to have pages that showed only certain home blocks. Basically they needed the same layout as the home page, but needed to have different home blocks than the ones on the home page.
To use them first think about the home blocks you’d like to display together. Then edit them individually and look in the right hand column for the portfolio group box. You can add them to an existing group here or create a new one. Add them all to the same group and update/publish them.
Now you can create a new page just for them. There is a portfolio group template you can use. It has a spot where you can specify the portfolio group displayed on the page. Update the page and that’s it.
This typically works best when you have one group for the home page and then one additional group per page for the other pages where you need this layout.
If you have any questions or trouble, let me know.