Mendeley's Groups feature is available in Mendeley Desktop and Mendeley Web. There are three types of groups with different levels of openness and sharing.
Private groups
Share documents and references with others by inviting them to participate in the group. With the basic Mendeley plan, private groups can only have 3 members and 100 MB of storage space for group resources. These private groups are visible only to members, who can share PDF documents and notes.
Invite-only public groups
These publicly visible groups have a restricted membership list, which is controlled by the group's owners. You can upload PDFs, but they will only be available to others if they have access through their own institutions or if they are open access articles.
Open public groups
These are publicly visible and everyone can contribute citations. Great for open discussion groups around any subject.
