I managed to put authors in diferent usergroups.
For their forum posts I modified postbit template with this condition, that checks the usergroup they belong to and displays their google banner:
<vb:if condition="is_member_of($post, 10)">
Still,
for CMS articles, this conditional doesn't work in vbcms_content_article_page.
What conditional to use, regarding author's user id or usergroup id, so I can show his adsense code in his articles only?