Once a document has been shared with a private group, all members have the ability to download and read that item. Mendeley further enhances the collaboration process by offering a number of features to allow you to work together with other group members.
By accessing the shared document through the private group you are able to highlight or add notes to draw your collaborator's attention to a particularly relevant section, or to provide further details as required.
Each group member is assigned a unique color to help track who has made particular comments or highlighting. All comments will also contain a record of the user who made it and when it was made, to help you make sense of the notes.
The notes tab of the right-hand panel in Mendeley Desktop provides an overview of all the comments made within a document, and also allows you to jump to the specific location a collaborator has identified when writing their note.