That and an invitation from someone already in the network. This is a good concept.

Its a good way to prevent sybil attacks and let the network grow slowly. Senior participants have only 1 invite per round, and some of the invites you give are not validated. So we are growing super slowly.
Overall the system is quite nice. And is something new! i'm so bored of the same ideas again and again in the crypto - altcoin space.