To add a new user or edit already users go to 'Settings > Users' in the menu. You can assign different roles to a new user.

Owner has an unlimited right to administrate the project, add new users and assign any user roles (Editor or Author).
Editor can add new users, assign any user roles (Editor or Author), see stats for all videos and edit them if it is not limited by the author.
Author may work and see stats only for self-uploaded videos. Authors can't add new users.