I'll use DotNetNuke as an example. When a user registers on a website, they give thier email and that gets stored in a users table along with thier password. On the cell phone, you can store your email and password and then use that when the phone sends the gps information to the website.
When the phone contacts the website, the website will need to take the password and encrpyt it and compared the encrypted password to the value that's in the user's table in the database. I beleive there is a method you can call in DotNetNuke to encrypt the password. So you don't really need to add another table, just use the users table and of course the tables that are part of the cell tracker application that I wrote.
Nick