private beta “real” user verification onboarding and vouching system

Hi All – a question on private beta “real” user verification onboarding.
To verify a new user is real, have you ever seen a system that required vouching or p2p verification by another human or existing user? Solution is partley to create exclusivity, partly to assure real ness of users. Example: new user downloads app > new user must get verified and enroll other pre-existing user to vouch for them > other pre existing user vouches for them > they are now verified.

• early gmail gave its users a handful of limited seats to invite friends. This had a cool viral hook but not much of a verification for real user.
• early asmallworld gave users an alottment of limited invites to bring in friends. new users had to be vouched by existng user.
• superhuman allows waitlist users to skip to front of line if they have a referrer who is already using superhuman

Welcome any thoughts on this or examples, failures or successes. Thank you for considering.