If condition which returns true if the creator of a post belongs to usergroup

[GrenadeSpam]

For our forums (VB5), I'd like to give staff members a differently coloured backgrounds in their posts. To this end, I'd like to add the HTML class "staffPost" to each post made by a staff member (usergroupid 5 or 6).

I've spent an entire afternoon searching for the right condition, but can't seem to find it. Any help would be greatly appreciated.

In pseudo code:

<vb:if condition="owner of this post belongs to usergroup 5"> staffPost</vb:if>