You will need to create the library using the Netbeans Library Manager. Go to Tools→ Libraries, then select the button New Library...and create a new class library. You can add jar files using this method, not a Netbeans project.
To add a Netbeans project:
In the Netbeans Projects window, right-click the folder Librariesin your project and select Add Project..., then go to the Netbeans project that you would like to use. You will need to do this for each project that you would like to use.
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