Uploaders and users with higher user classes get invites. You also get some minor perks if you donate, one of which is one invite. It's $10 to get access to those perks. So technically, you could send a friend 10 bucks and he could invite you in.
I don't think I've ever seen this forum mentioned on any recruitment thread like you would with private trackers, so a direct contact is probably the only way in. They've done a pretty good job at staying out of the spotlight since the domain change. It's a pretty underrated place.