You can use Groups to arrange references in ways that are meaningful to you. However, EndNote allows you to also create Group-Sets to organise your groups. Your groups and group sets will appear in the left-hand panel in EndNote under My Groups. 

To create a new group set:

1. Click on the groups menu
2. select create group set
3. a new group set will appear in the left hand panel
4. give it a name that is meaningful to you 
5. to add a group to the group set either click and drag a group onto the group set or click on the group set name and create a new group from the groups menu

When the list is collapsed, the number next to the Group Set header reflects the total number of references found in each group. This number could include duplicates (references that are included in more than one group).