There are 2 ways to buy tickets before the general public. As you may know, music festivals in general are an exception as they usually offer an early bird system (i.e. tickets go on sale early to the general public and if you buy early, tickets are cheaper!)
1. Buy tickets at the official advance sale (or lottery):
Usually for popular events, they have an exclusive period for their regular fans, called fan club members. They pay an annual fee in advance (i.e. around 5000 JPY per year - some have a monthly plan) and get earlier access to the pre-sale. If the events are very popular, they will organise a lottery process (i.e. you apply for a ticket and only a selected number of applicants can win tickets). Unfortunately, this process is often only available in Japanese.
2. Presale (first come, first served) from a ticketing company:
After the original fan club pre-sale period, there is usually a basic first-come, first-served pre-sale period run by Japanese ticketing companies. This period exists because of the competition of each local ticketing company - so it is hard to guess which companies will have this period. Usually, they will charge more commission (around 500-1000 JPY per ticket excluding ticket printing fee, shipping fee and payment gateway fee) from a ticket buyer during this time, in exchange for offering a lower commission to the event organiser,