Click into the Group By menu to select a field to group the plans by in the Portfolio grid.
Here is an example of the Portfolio list grouped by Business Unit.
You can even group plans in your Portfolio my multiple fields. To apply multiple groupings to your Portfolio grid, click into the Group By menu and select the fields to group the plans by in the Portfolio grid. The first field selected will be the top-level grouping, the second field will become a child grouping of the first, and so on.
For example, here is a Portfolio list grouped by Process Step, then by Schedule Health, then by Created By. You can see how the groupings are hierarchial.