EndNote has 3 main methods to organise your references:
A Custom Group allows you to add references individually to a group.
A Smart Group is compiled automatically based on the parameters set when creating the smart group. As it is constantly searching your library, there is no need to add references manually.
The references in your library that match your criteria will be automatically moved into your smart group
Tags are a new feature of EndNote 21 and provide a visual organisational method.
First, create the tags you wish to use. These will be visible in the left pane and when clicked, will display all references with that tag.
Adding the tags to references
Alternatively, drag and drop your references into the tag folders in the left-hand pane.