An invitation code means pretty much what it says. Another active member must send you an invite, thus providing you with a code for registration. So unless you can get a referral, you can't register an account.
There are tools to recover lost passwords from web browsers (try a Google search). You might be shocked to find just how much data is still there. If you've completely deleted this data, don't have a back-up and never kept a copy of your password, you are probably out of luck.
I have an account/password, but it stopped working long ago for no reason I can think of. Best guess, either keeping the costs of the site down or some filtering prevents e-mails that aren't handled by Chinese providers. That would fit with the narrative of the locked-down internet of China these days, but I'm only able to speculate.