This is just speculation. I had argued this some on a forum, those who felt like commenting did not necessarily agree. This has been altered some based on that discussion.
I speculate that the Phoenix people were the ones to have built Jusendo for the purpose of Saffron and that Jusenkyo was a side effect/add-on of the building of Jusendo.
The phoenix people evolved from run-off of a spring at the top of phoenix mountain that is cursed like Jusenkyo but not part of Jusenkyo. I speculate that at one time in the past they went searching for the source of their spring. They found an underground water vein that had magical properties but did not alter people like their spring did. They experimented with the water eventually producing Saffron. To make his ascension easier they drill another mountain that has the underground water vein beneath it, Jusendo, and build the pool for him and the taps. The building of it happened around 4000 years ago according to one the characters in volume 37. The Phoenix tap is used for Saffron's ascension, the dragon tap though also has a purpose for them. Namely while experimenting with the water they noticed that if something drowns in the water the pool gives the next person that body. This is very useful to them so they create Jusenkyo with the water from the dragon tap. We know they use Jusenkyo since at least two of the characters were cursed to turn into humans so they could blend in with humans. We also know that the phoenix people create new Jusenkyo pools since they created the pool using Akane as the template.
Somethings from the manga that support this:
-Only known usage for Jusendo is Saffrons ascension.
-The Phoenix people have the most reason to look for the source of the water -namely to figure out why they are changing.
-The Phoenix people are the ones who know the history of Jusendo.
-The Phoenix people have more of a reason than any other people in the area to use and build it.
-There is no evidence of any people using the pools as long as the Phoenix people. The Musk for example can only be shown having used it for less than 1500 years since that is the age of the girl spring that they used to transform animals. Though it is possible that there are other girl springs since there are at least three male springs we know about - pious man, boy and the one that can cure Ranma.
-The map at one point in history was likely the phoenix peoples. After all they knew about the map and that when submerged in the water it would show the safe way into Jusendo.
-The Phoenix people's culture and way of life is based on it's use.
-both Phoenix mountain and Jusendo are trap ridden.
Why did the Phoenix people need the map if they built it? Simple knowledge is often lost in intervening years and while they knew where the taps were they no longer knew the safe entrance.
How did the guide get it? Who knows, maybe it was stolen or maybe the guide had worked for the phoenix people the last time they had need of the map (100 years or so ago) and one of the guides in the in between times switched to working for someone else.
Why did the safe entrance say "employee's only"? Originally the map was needed to find the safe entrance but in time someone other than the creators/phoenix people found the mountain. They located the safe entrance, or created a new entrance, and labeled it employee's only.
That they chose to make a dragon tap and the Musk leader being part dragon is just a coincidence. Dragons are common in all cultures within real life. There are other similar coincidences in the manga, like the ugly bird that was on Kuno's head being called a phoenix as well.
It is also possible that Phoenix mountain did not have the same source as Jusendo since there was also a Jusenkyo type spring in Japan but it had dried out by the time Ranma found it.