Users with specific Roles, Online Now

I think for certain types of websites, a plugin which displays a list of users who are currently Online would be great, but the really great part is to only display a list of users from Admin Specified ROLES.

So people with the role of "Contributor" or which ever, would appear online, in a widget, or a thumbnailed list.

For me I have people helping out on my website by answering questions, so they have specific roles they essentially work for me, id like my regular members to see when they are online

thanks!