So the HackerNews invite code for JamLegend is located in our DB. Our strategy has been the following:

A) Create one invite code, send that to a lot of people. B) From the people who sign up, give each new account an invite code itself, so that person can sign up. You can assign a random string or set a class for 2nd degree invites. C) Repeat step B, and adjust invites accordingly to the amount of growth.

A prerequisite for Step B is making sure that you have invitations built into the system, so that people it won't be ignored.