I just got "invited" a day or so ago. That's what you have to do, wait until you get an invite.
It says on the link below that "WGT NATION is free to join and is by invitation only for WGT Legends and players Level 90 and above, as well as a limited number of other top customers invited by WGT."
Of course I'm not even close to L90 or Legend. Looks like you've been here longer than I, have a better average, are at a higher level, have nice equipment (you've spent $$), so I have no idea what the criteria is.
https://www.wgt.com/nation/Default.aspx