Group Leadership Roles:

A group is a publishing organization, and the people above run it.

Admin has absolute control over the group -- they can manage publication schedule, invite more people into the leadership, fire them.
Editor can manage the queue (actually publishing diaries to a group's home), but has no ability to hire and fire.

Member can just queue things, but can't actually publish to the group's home page.

For a more specific list of permissions, mouse over the question mark on the "Role" header.