BuddyPress.org

Try something like this:

function paragbhagwat_message_filter( $message, $group, $requesting_user_name, $profile_link, $group_requests, $settings_link ) {
    // Do something with $message here.
    return $message;
}
add_filter( 'groups_notification_new_membership_request_message', 'paragbhagwat_message_filter', 10, 6 )