Administrator is a folder/channel creator. They can add and remove members, rename and move channels, edit channel description.
Member is a folder/channel member who is not allowed to remove members, rename channels, or edit channel description. A member can suggest the Administrator do all these actions though and it is up to Administrator to approve or decline them.
Guest is a user who was invited to a private folder subchannel without being a member of the private folder. Guest user has the same rights as a Member within the subchannel he/she was invited into.