You can add references to a new or existing custom group at any time. You do not have to worry about copying the same reference to a group more than once; EndNote will not allow duplicates (based on unique EndNote record numbers).
To add references to a group:
First, select the references you want to add to the group. You can do this in many ways, for example:
Click on an individual reference to select it.
Display a list of references (such as search results), click in the list of references, and then go to the Edit menu and choose Select All.
Hold down the Shift key and click on two references to select a range of contiguous references.
Hold down the Control/Ctrl key and click on individual references to select them.
Add the references to a group. Several methods are available:
Drag and drop the selected references onto an existing custom group name in the left panel of the Library window.
From the Groups menu, select Add References To and then select either the name of an existing custom group or Create Group.
Control+click or Right-click on the reference list to display a menu, select Add References To, and then select either the name of an existing custom group or Create Group.
How do you know which references are not included in any custom groups?
The automatic Unfiled group includes references that do not appear in any custom groups.
The Unfiled group may or may not include references that are part of a smart group. Right-Control+ click on Unfiled to display a context sensitive menu and toggle the setting Exclude Smart Groups in Unfiled / Include Smart Groups in Unfiled.