Creating a People column allows you have a column picker that is automatically populated with the people in your document and workspace folder. In other words, you'll see all individuals who have access to, or have been invited to access, your doc as choices without any fancy programming. You can refer to people in a Coda doc by @ mentioning them.
If you want to remove people from a people column, you can right click their entry to reveal the options menu and remove them.
People list in the Dropdown
So how does the list of users get into the drop down? There are few ways to make it faster to select people that already have access to the doc.
The MyDoc folder is the special folder that you get when you are added to a workspace. This is a private doclist for that others in the workspace cannot see. If you explicitly share a doc from the MyDoc folder then that person will now appear in the dropdown list.
If the doc is in any other workspace folder then the people list comes from the members that have joined the folder. You will also see people that have been explicitly added to the doc as guests through the sharing dialog.
When adding a doc to the workspace there is a "show more" option in the drop down. This will enable you to browse through all the workspace members and select a person that hasn't joined the folder yet or been shared the doc.