You could use something like this in your header template
<if condition="is_member_of($bbuserinfo, 1)">
<br />
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<thead>
<tr>
<td class="tcat" style="color: white;">
Unregisterd User
</td>
</tr>
</thead>
<tbody>
<tr>
<td class="alt1" align="center">
PUT YOUR TEXT HERE
</td>
</tr>
</tbody>
</table>
<br />
</if>
Where $bbuserinfo, 1 is change the number to the group you want it to be displayed to