Groups are… well, groups of people. They can represent an organisation, a team, or just a bunch of people of similar interest.
The primary goal is to provide the ability of creating team events; ie. events that are visible/relevant only to the team members.
These are public. Anyone can join without approval.
Closed groups are discoverable on the groups interface, but all its details (like members and events) are hidden from non-members.
Secret groups are completely hidden from non-members. Users can only get in by means of invitation.