In organization settings, you can invite your colleagues or friends. This is the only place where new users enter the system.
The three icons determine user rights. They are the same in all user management settings. When the book icon is green, the user can read projects in the organization. When the pencil icon is green, the user can write to projects in the organization. When the cog icon is green, the user can manage the organization - this means adding new users, edit settings and access anything in the whole organization. The manage right gives you a full control of the whole organization, its projects and collections.
The settings of the user rights in project settings are same, they give users rights to read, write and manage the project and its collections.
In collection settings, the icons give users rights to read and write data in the collection and manage collection settings.