I had a mod for this a while ago, but I'll be damned if I can find it or even think of the search term to hunt it down.
Basically, the users online list would have the usernames colored based on their group membership. I tried adding style data to the group itself, but that only impacted how the groupname itself appeared, not the username.
This was great for highlighting my mods/admins.
Anyone got a rundown on how to do this? I've fiddled a tidbit with the templates, but all I've managed to do is break stuff.