Invisible - don't pay it. That is NOT a fine, it is an invoice.
If you refuse to pay and they wish to enforce it, they will have to sue the driver in the civil court. They have no idea who was driving.
They will have to prove the driver was in breach of contract. The driver can only be in breach if he or she were aware they were forming a contract.
If you cannot remember who was driving, and they can't provide photographic evidence, they have fell at the first hurdle.
There are template letters on the internet for this sort of case.
Mostly it is too expensive and too time consuming for these people to pursue it in court and there are too many defences for the motorist for them to bother.