Complex sentence: There were 12 participants in the first program, but it grew rapidly in the following years as it provided the only training course of this kind in Japan at that time, and participants assembled from all over the country, from Kyushu to Hokkaido.
Split into simpler sentences: There were 12 participants in the first program, but it grew rapidly in the following years. It provided the only training course of this kind in Japan at that time, and the participants assembled from all over the country, from Kyushu to Hokkaido.