This is kind of off-topic for Meta, imho.
These are questions for their team.
But by the looks of things, it is indeed a Discourse site and you can sign-up via the Login button on the top-right.
(And as an aside - I note it appears to use my theme component heavily! 
)
@merefield
So you don’t know something about ZBrush?
And where is their forum?
Hidden to anon? Best of luck
I see, discourse is a forum platform like wordpress for websites,
but forum component, right?
Discourse is a free open source forum platform you can download and install on your choice of Ubuntu VPS.
See: standard install
You can install third party Theme Components and Plugins (like mine)
Sites are not centralised. They have their own management, so for access issues on specific sites you need to contact them there.
Got it, thanks for an explanation!
Have a good day
You too @meded90 !