Invitation codes have been available since early December 2020; in one of - if not all - of the actual 4KXX project threads on here.
The second post of this thread has stated there are invitation codes available for TSWT forums in the three main 4KXX threads since early January too…
OT.com Page for 4K77 : OT.com Page for 4K80 : OT.com Page for 4K83
^ There are invitation codes to the TSWT forums in each of the Opening Posts of these three 4KXX project threads.