There is not a single alert, where a group admin receives an alert for join requests! To be honest, that´s a no-go since many will have several groups, even those with only themselves as user in a closed group. They won´t check the group itself regulary since they will expect - like I did - there will be an alert send to the alert system. The only info about a join request is available in the corner at the very bottom, where noone will ever look for it. Please change that.