EndNote Basic does not include an annotation tool and does not allow sharing of PDF files between group members. You would need an EndNote Desktop (licensed) version for that. But there is a great free alternative!
Download Adobe for Google Drive for free to annotate and mark up PDF files.
Once installed, when you open a PDF file in Google Drive you have the option to open with Adobe Acrobat for Google Drive, as shown below:
You can see an example of a marked up PDF below: