Watch My Domains SED supports categorizing domains into groups. The category names are listed in the left panel. You can use the toolbar at the foot of the category table to add, rename and delete categories.
To add domains to a category, select the category in the left pane and then use the 'add domains' (or the 'Upload Data') option to add domains to your portfolio. This will add the domains to that category. You can use this to add even already existing domains to a category.
You can also use the 'Bulk Tool' to add a group of domains to a category or to remove a group of domains from a category.