Problem with conditionals

[dismas]

In my postbit template, I have:

         <if condition="is_member_of($bbuserinfo, 5,6,7,15)">
              <div>
                  <if condition="$post['field5']">
                      a.k.a.: $post[field5]
                  </if>
              </div>
         </if>

Usergroup 15 is a club member's only usergroup. Basically, when someone joins the club, I add them to a subscription which puts them in 15 as the primary and 2 (Registered Users) as the additional usergroup. I'd like this to be visible to members of 15 but not those who are only in 2.

So do I have to change the way my subscription is set up or the way I have the if condition set up?