<p>What's the best way to get a list of users that are assigned to a SharePoint portal?</p>

It depends on exactly what list of users you want.
All users that exist in a SharePoint web (but don't necessarily have permission):
SPWeb.AllUsers
Users that exist in a SharePoint web but have been granted some permission:
SPWeb.Users
Users that exist in a SharePoint site collection (but don't necessarily have permission):
SPWeb.SiteUsers
Note: I am assuming you are using WSS 3.0 / MOSS 2007 but this should work for WSS 2.0.
Also look at:
SPWeb.AllUsers vs. SPWeb.Users
SPWeb.AllUsers vs. SPWeb.Users vs. SPWeb.Groups