In the Portfolio Dashboard, click for the following dropdown. Select 'Assign projects to groups'.
The group I've created called My New Group is currently empty. I know this because the included projects column show all the projects within my group, and there are none in there presently. The projects I want to be in my group, I simply drag them from the excluded projects list and drop them in the included projects list. The groups is saved as soon as the drop is made.
Return to the Portfolio Dashboard and change the dropdown view to your group name. The table will automatically show only the group of projects configured in your group.