Another proposal is to allow members to join, but only if sponsored by two existing members, or require some verification. That creates a simple choice - if you're new to the community, you may need to prove you are a human (a photo with your face, the username and date on a piece of paper would be sufficient), to avoid sock-puppets. Otherwise, you can just have two existing members invite you, no need to prove or provide identity.
That doesn't verify anything. Once I have control over two accounts (e.g. mine and my friend's; or mine and mine because I had the skill to find some photo of a human in that thing called internet), I can start inviting myself over and over again to sign up thousand times.