Come and have a look at what we're doing at the moment, we've just finished race 2 of the new season. This particular series is Honda based but this season is very different with GT500 spec and rules.
https://www.gtplanet.net/forum/thre...e-2-fuji-f-5th-july-accepting-entries.305570/
Below is our HQ as such, this series has just finished. The friendship borne from these 2 leagues has seen us successful at the GTP Endurance Series.
https://www.gtplanet.net/forum/threads/the-sunday-cup-lotus-club-gt.238234/
Both these threads are on the page you posted on.
Best thing to do, find a thread/series that looks interesting to you, read through rules/regs and as much of the thread as you maybe can. (remember the OP is usually updated by organisers but the first pages could be very old and not specific to their current series)
If you like the look of it, introduce yourself to the organiser/staff in a new post or how they may have instructed you to in the OP (PM or PSN Friend Request)
A series may require you to set a bench mark time (if it is large, to join you with similar skilled drivers) or just join they're practice nights/community club practice lobby.
If you definitely like it, FR all people in the club to help room stability, get practicing, tuning (if series allows) and get racing.
It's the best GT can offer. Humans!