Join PTP, RED etc and get access to the invite forums. FYI you won’t get an invite from a regular user, it takes years to get to the user class that gets invites
To get access to the invite forums on RED or PTP doesn't take years. It takes 4 weeks max. 2 weeks for RED and 4 for PTP. You have to meet the other requirements besides the required time tho.
you either get invited by someone you know or you build your reputation on a different private tracker and gain access to their invite forum where BTN have an official invite thread.
BTN is currently not recruiting and hasn't for years. In fact they just lowered their usercap with no signs of official recruitment. Your only way is for you to have a friend in it, for him to be high enough rank and wait for new years to buy an invite.