[Mythotical]

use if subscription adds user to secondary usergroup:

<if condition="$bbuserinfo[membergroupids] == X">

<else />
Adsense code
</if>

use if subscription adds user to new usergroup and no secondary:

<if condition="$bbuserinfo[usergroupid] == X">

<else />
Adsense code
</if>

EDIT: the else just states if the member is not part of the usergroup specified then ads will show.

Cheers
Steve