To log a user into Sitellite, you simply need to pass any page request correct username and password GET or POST parameters. For example:

http://www.example.com/index?username=some_user&password=fake_pass

Or via an HTML form:

<form method="POST">
    <p>
        Username<br />
        <input type="text" name="username" />
    </p>
    <p>
        Password<br />
        <input type="password" name="password" />
    </p>
    <p>
        <input type="submit" value="Enter" />
    </p>
</form>

Then on the receiving page, you simply need to check whether the user login attempt was successful.