[Paul M]

You should actually use ;

<if condition="is_member_of($post,6)">
Something here (if poster is in admin group)
</if>

Also, you can specify more than one group, e.g.

<if condition="is_member_of($post,5,6,7)">
Something here (if poster is in group 5, 6 or 7)
</if>