Within an individual project, the ability to edit or add people within a delegation is defined here. A user has two different sets of rights to a delegation: "Access" which defines whether or not the user can change the delegation's setup, plus "People" which lets him add, edit or delete people within the delegation.
A user gets rights to a person within a project via that person's delegation and/or role. The user needs only one of these, meaning that if he has rights to a person via the person's delegation then the rights he has to people with that role are irrelevant, and vice-versa.
Rights to view a person are determined solely by the user's rights to that person globally. To edit a person within a project a user needs both global edit rights to that person as well as edit rights within the project.