Just add a new column to the entry and repeat tables called "email" or whatever. MRBS will automatically pick this up and add it to the booking form.
No, it has changed slightly in MRBS 1.4.11. You need to modify the function create_addresses() in functions_mail.inc. At line 809 add the line
$to[] = $data['your_field_name']; // change the key to match your field name