Mendeley now only supports private groups, for sharing references and full-text files.
To create a group, click on the "Create Group..." link under "Groups" in the left sidebar:
Type in the name for your new group:
You can then invite members (if desired). Private groups can have up to 100 members.
When searching for papers in Mendeley, you can learn a little bit about each paper's popularity with users. The box on the right side of each description shows brief statistics: