On a Group, how can I retrieve the dates on which a member was added and by whom?

I'm able to read the stream from a group but I'm particularly looking for the posts where it's mentioned that so-and-so has been "added by" a member and on which date. Eg:

John Smith and 12 other members were added by Alexander John. Unlike · · Unfollow Post · Saturday at 4:02pm

Unfortunately, I can't seem to extract the above info using 'method'=>'fql.query' on the stream table. Even if I manage to find the Ids of these "added by" posts, when I run it as a query, i get an empty array. For example:

$sql = "SELECT post_id, permalink, created_time, updated_time, actor_id, message, attachment, likes, comments FROM str开发者_运维百科eam WHERE gid=$gid AND (post_id='243493399035757' OR post_id='243503932368037')";

If this info shows up on a Group's stream, then it has to be coming from somewhere, right? So, does anyone know how I can get/extract these "added by" posts using FB's API or otherwise??