Keep these points in mind as you work with groups:
You can create up to 5,000 custom groups and smart groups (combined) in a single library.
You can create a maximum of 5,000 group sets in a single library.
You can add a reference to a particular group only once (based on the unique record number). A single reference will never appear more than once in a particular group. However, you can add that reference to any number of groups.
When you are viewing a group of references with selected (highlighted) references, and then switch to viewing All References, the references are selected in the All References view until you deselect them.
You can drag references from one library directly into a group in another open library. The references are copied to the second library and into the target group.
Removing a reference from a custom group does not delete it from the library. It removes the reference from the group subset, but the reference still exists in the library.
Deleting a reference from a library also deletes it from all groups in that library. The reference no longer exists. Highlight the reference, go to the All References view, and then select Move References to Trash from the References menu.
When you delete a reference from a smart group, EndNote also deletes the record from the All References group, with no warning.
Once you move a reference to the Trash group, it is no longer officially in your library (it will not appear in any other group or reference list, it is not included in your reference count, and it cannot be searched or cited). However, until you select Empty Trash from the References menu, you can still drag the reference from the Trash group back to your active library.