This action runs when a user membership request for a group is approved.

 do_action('pm_user_membership_request_approve',$gid,$uid);

Simple example

function profile_magic_action_on_request_approve($gid,$uid)

{

//do something with the user id and group id when a user membership request is approved.

}

add_action( 'pm_user_membership_request_approve', 'profile_magic_action_on_request_approve');