kodpkd - sounds like your Jag has a key with a built in remote, like the key fob that comes with the Focus.
THAT is easier to program to the car, did an extra for my Focus and all you need is ALL the fobs you want to use on hand at the same time.
The Focus KEY on the other hand, has an RFID chip inside, and the car's computer has to recognize that chip to allow the car to start.
The programming assumption, as mentioned above, is that anyone with TWO keys already in the system is likely to be an owner or authorized user - so an additional key can be added by that person to the list.
If it was easy, it wouldn't be much of a "security" system...