You could edit the FORUMHOME or header template and use the following conditional:
Code:
<if condition="is_member_of($vbulletin->userinfo, X)">
<!-- HTML CODE TO DISPLAY TO THIS GROUP -->
</if>
Change
X to the usergroupid number of the usergroup you want to display the message to. There might be a mod about somewhere.
Also this feature is available in 3.7 if you are prepared to wait till the stable version is released.