Alert when a groups number of members changed

I can see in the message center and in the events tab, events such as Group (groupname) Gained xx members. However I cannot find a way to alert on this member change.

I have checked all the alert fields I could think of and I cannot find it.

This is imperative for a map process I am implementing, using status from a group to get map status. so I need to know when a group loses or gains a member so I can add it to the map.