ActiveResource--authenticate with a user table in another app

I have a main app that has a users table. Now I want to make another
app that supports/extends the main app. I would like to authenticate
the supporting app with the main app user's table. How do I begin

Create a User activeresource model in the supporting app?
Copy the authentication code in my main app into the supporting app?

What is the best way to go about doing this?

Thanks for any help!