Need help checking the $post[membergroupids] array in postbit, please

[Spinball]

vB 3.8.2, I need to check in the postbit whether the poster of each post is in a certain usergroup.
I'm sure I'm nearly there - $post[membergroupids] contains the secondary usergroups - but it's not working.

<if condition="in_array(31,array($post[membergroupids]))">
		<div class="specialusergroup">
</if>           
	$post[message]
 <if condition="in_array(31,array($post[membergroupids]))">
		</div>
</if>

I'm looking at posts by people who are secondary members of usergroup 31 with no div tag being placed round their messages.
What am I doing wrong?