From what I understand is that you just want a user which can login in your app to be selectable rather then all persons known in your application. To achieve this you could use the "Account" entity which is a generalization of the System.User entity. In case you don't use the Administration module you could directly connect to the System.User object. But I would rather go over the Account object.
Maybe I am better off going about it this way. I created the following entities... So through this I can reference many people as part of the project which is great as my first goal is to create a "project directory" of various people and their contact information.
The question is how do I assign one single person as the "Project Owner" which I want to show on my data grid as a sortable and filterable column. I need to take each of these "person" objects and tell my Project table what they are in relation to the project.