Groups allow you to share tasks for a given project, party or anything else you're doing with a group of people. Groups are managed by an 'Organizer' who has the power to add and delete members of the group. Groups can also have more than one Organizer. 'Members' have the ability to change tasks (add them, cross them off, commenting on them etc.), as do Organizers. 'Guests' can see what goes on in a group, but they have no ability to change anything.
Seen another way: