Question: what is the easiest way to add a column to a list or document library in SharePoint 2007?
Answer: navigate to the list, from the Settings menu, choose Create Column.
Question: how do you add an existing Site Column to a list or document library?
Answer: navigate to the list, from the Settings menu, choose List/Document Library Settings, scroll down to the Columns section and click Add from existing site columns.
In the beta versions of SharePoint 2007 there was an easier way to add an existing Site Column to a list or document library because there was another menu item in the Settings menu: Add column from existing site columns. For those of you who are nostalgic about the beta's of Office 12, and want to have the menu item back: I've created simple feature to add the menu item:
<?xml version="1.0" encoding="utf-8"?>
Title="Add column from existing site columns"
Description="Add an existing site column to store additional information about each item."
Here's a screenshot of the result: