Join up is a term coined by Monty Roberts. He did not invent this process but he popularized it and explained it in comprehensible terms. He has tons of videos out. I am not his disciple by any means, but you can learn something from him, amounst others.
Basically, you put a horse in a contained situation, then apply pressure on him , making him run and because he cannot run AWAY, he runs around you. As he looks for a way out, a way to get away from the pressure, he will eventually "ask" in horse body language if you will please let off the pressure so that he can stop running and come in to "join up" with the "herd", which in this case, is you.
Monty observed how in horse herds dominant horses sometimes disclipline youngsters by driving them out of the herd and not allowing them back in. The youngster will defer to the dominant horse by lowering its head and licking its lips and such (actually, just a common stress releiveing motion horses use a lot). When these signs of acceptance of your dominance are seen, then you allow the horse to come in and stand in safety next to you and follow you around. Supposedely, after this, the horse will view you as its' leader. One should not have to do this every time they ride, but if the horse tries to take back the leadership (which will happen IF YOU don't be the leader he needs you to be) , then you would do join up again.
Mind you, just doing join up isn't going to make everything else just fall into place, but it may start you out on the right footing.
Would be fun if you video taped it.