[Lynne]

Did you look at the API for is_member_of?

is_member_of (line 384)
Works out if a user is a member of the specified usergroup(s)
This function can be overloaded to test multiple usergroups: is_member_of($user, 1, 3, 4, 6...)
boolean is_member_of (array $userinfo, integer $usergroupid, [boolean $cache = true])

• array $userinfo: User info array - must contain userid, usergroupid and membergroupids fields
• integer $usergroupid: Usergroup ID to test
• boolean $cache: Pull result from cache

So, as seen, it needs more info passed to it. You may have to do a join on the thread query to get the usergroupid of the user also.