Here is the Japanese Wikipedia page on this group, which I was not familiar with. To recap briefly what it says:
The group was founded by one Sadao Inai (1905-1988). He suffered from cancer of the pharynx and prayed to the god Ooyamanezu-no-mikoto and healed. This lead him to found this organisation to “show the world the existence” of said deity.
In 1952, things took a turn for the worst. At a time when the number of believers was steadily rising, a high-ranking member of the organisation was accused of raping one of the female believers. Furthermore, a sort of internal coup d’etat was staged by none other than Inai’s own wife.
These troubles were however dealt with, but the people who opposed Inai were kicked out of the organisation. The went on to found some of their own sects, which number up to 20.
To avoid similar strife, the structure of the organisation was changed to be more centralised. This allowed it to get back on its feet. It is said that between the mid-70s to the mid-80s the number of adherents grew from 50,000 to 800,000. Many of them converged to the area of Makita, where the church is based, which led to the construction of many buildings and clashes with the local residents.
Inai’s second in command, Hideko Mori took over after his death. She passed away in 2002. Since then, the leadership has been split between two persons. They have 24 centers across Japan.
Inai’s popularity was due in part to his sense of humour.
While the current number of followers isn’t clear, from the number and scale of buildings they own, one can surmise that they are many.
Prayer to Ooyamanezu-no-mikoto is at the center of their practice. Each morning and evening at 6:00, believers must pray. They are made to purchase radio clocks so that they are not even a second late for the prayer. Once a month, they gather to hear preachings. They value social obedience; for instance if one cannot do the evening prayer because of work, waiting after work to pray is okay.
Anyway, that’s what’s on Wikipedia. It seems largely Shinto-based, although the efforts put towards recruiting new believers is typical of new religions. On the dodgy front, they seem to be able to suck substantial amounts of money from their members, if their buildings are any indication. Also, they advocate miraculous healings and such, as a matter of fact, this appears to be their central message: “if you pray regularly to Ooyamanezu, he will heal you like he healed our founder.”
Like I said, I wasn’t really familiar with them, so that’s just what I can surmise from reading a bit.