Email alerts on BuddyPress group’s event creation

I think the following code snippet should do what you’re requesting.

add_action('em_event_save', 'my_em_group_event_save', 10, 2);
function my_em_group_event_save($result, $EM_Event) {
    if (is_object($EM_Event) && $EM_Event->group_id) {
        $group = groups_get_group( array( 'group_id' => $EM_Event->group_id ) );
        $post = get_post( $EM_Event->post_id );
        $subject = sprintf( 'New post in %s: %s', $group->name, $post->post_title );
        $message = sprintf( 'A new event has been added to the group "%s". To view the event, click the link below:%s%s', $group->name, "\n\n", get_permalink( $post_id ) );
        $headers = array( 'Content-Type: text/html; charset=UTF-8' );
        $recipient_list = array();
        $members = groups_get_group_members( array( 'group_id' => $group_id ) );
        foreach ( $members['members'] as $member ) {
            $recipient_list[] = $member->user_email;
        }
        wp_mail( $recipient_list, $subject, $message, $headers );
    }
    return $result;
}

You can use the WPCode plugin to help you add the code snippet on your website.