Very true! "Join up" is not something you teach a horse. I don't even like using the term "join up". Think of it more as communicating to a horse using "horse language" (not human language). It's just like SamsonB's post above...I sure didn't understand what it meant, but I know it's a foreign language!
Horses don't understand humans, in fact the only thing that they naturally comprehend about humans is that we are predators. As humans, we have the ability to change the way we communicate. Horses cannot do that - not in a million years will they learn how to speak in our language. We must speak to them in their language.
Parelli is good. I've used his techniques (love the 7 games!). But in this situation, I would recommend someone else. Go to http://www.marvwalker.com and send him an e-mail requesting The Bonder. It is about six pages of text that describes exactly how to communicate in "horse language". Read through it 10 times before you try it with your horse. You have to have it ingrained in your mind so that you are prepared for everything your horse will tell you throughout the process. IT WORKS!! I promise! I use The Bonder as the basis for every horse I work with. Using The Bonder has made everything else so much easier.