By mistake I did not add my girlfriend on my booking for WizzAir with discount. Can I add her on the same flight, same date and hour with discount?
To use the WIZZ Discount Club, your girlfriend's ticket must be on the same booking as yours. Unfortunately you cannot add passengers to your booking, so you need to create a new one. But I assume that it will cost more so it's not worth it.
Can I add more passengers to my reservation:
Unfortunately this is not possible. You will need to create a new booking.