Mendeley offers three different types of groups for sharing information and collaborating, each with varying levels of permissions. See below for details.
Allow for sharing references, PDFs, and shared multi-user PDF annotation
Members can participate in group discussion
Mendeley's free version allows for 5 private groups with up to 25 members.
Require invitation or an administrator-approved request to join
Publicly visible
Allow for sharing references (citations, not PDFs)
Non-members can "follow" the group and participate in discussion but can not add new references
Membership is open to the public
Publicly visible
Allow for sharing references (citations, not PDFs)
Any member can add new references to a group
Adding References to a Group
Desktop: Drag and drop selected papers into the group listing under "GROUPS" in the left-hand panel.
Web: Select papers from your library, then click "Add to" in the bar over your library. Select "Group" and then the desired group followed by "Share" to complete.
References will be available to group members immediately in Mendeley Web or after pressing "Sync" in Mendeley Desktop.