To limit it just to certain user groups I'd recommend the following conditional rather than the in_array() solution posted above. This version ensures that secondary group assignments are checked too.
For a single user group (1 in this case)
PHP Code:
<if condition="is_member_of($bbuserinfo, 1)">
For multiple user groups (1, 2 & 3 in this case)
PHP Code:
<if condition="is_member_of($bbuserinfo, array(1,2,3))">