Domain authorisation in ASP.Net

Hi, I have a website that uses windows authentication to ensure that only people who exist on our domain can access our site.

VB Code:

<authorization>
        <deny users="?" />
    </authorization>

We do have, however multiple domains so what I would like to be able to do is allow only certain people from a certain domain to look at the site, kinda like this

VB Code:

<authorization>
        <allow users="MYDOMAIN/*" />
    </authorization>

I am unable to do the above, so any suggestions appreciated :)

Cheers
Mark