There are three types of groups in Mendeley:

1. Open or Public Groups - These groups are totally public and can be joined by anyone with a Mendeley account. Any member can add new references.
2. Invite-only Groups - These groups are pubclically visible, but can only be joined by users who are invited or apporved by an administrator.
3. Private Groups - Designed to allow small teams to work together, the full reference record (including PDFs, notes, and annotations) are shared between the members.

For more details, see the Groups Guide and the Private Groups Guide.