How to use BelongsToPermissionGroup() in a Responsive Template?

Hello!
I am trying to turn off the User Page button on the UI (Responsive Template: Shared/UserMenu) if the user is not in a certain permission group.
Looks like I need to use either BelongsToPermissionGroup() or IsInPermissionGroup(), but I can't figure out how. (I'll welcome any other way too!)

ATTEMPTS
It seems to need a prefix that defines the user. "IUser." (as per helpfile) doesn't work - seems that "IUser" is a type.
So how do I specify the Visitor, for example? Prefixing with "Visitor." doesn't work (even though that's a CTX tag, according to the 2008 What's New doc).

Example:

@if IUser.BelongsToPermissionGroup('TestGroup')
 {
 <li>Yes</li>
 }
 else
 {
 <li>No</li>
 }

Thank you!
- Sergey