If you add the Library's Proxy URL to your EndNote setup, you will no longer be prompted for a group ID.  Here are the proxy URLs you need:

EZ proxy address prefix:

http://proxy-hs.researchport.umd.edu/login?url=

Endnote users may also need sfx/open url:

http://sfx.umd.edu/hs?url=

If you need guidance to add the proxy URL(s), please contact EndNote Technical Support http://endnote.com/support/contact-support .  You can choose Submit an Inquiry or Call Us, we recommend the Call Us feature for the quickest response.

Also, I'm not sure if you were intending to search the Library's databases from EndNote.  If so, I would recommend a different approach.  You will be able to search more effectively by entering the desired database directly from the HS/HSL website.  Once you have your search results, you can export them directly to EndNote, along with the associated PDFs (if available).  The proxy URLs above will allow for the direct export of available PDFs.