Jack Russell's are known for being hyperactive. Unfortunately, that means that when one has company, these dogs' first instinct is to greet their visitors by jumping on them. Training them not to is not rocket science. It's actually very simple.

To train your company, instruct them that when your dog attempts to jump up on them, to simply take a step back. Depending on the dog, you may have to do this a few times for him/her to get the message, but I assure you, the dog will get the message. If the dog makes more than two attempts, simply say "Shh!" as you step back. The dog will quickly realize you are the alpha member of this relationship and will obey. The "Shh!" command must be authoritative; the dog needs to know you are the leader of the pack. What could be more simple?