I would have to agree with xux's points. You really do want to make sign-up as simple as possible. Possibly even consider authentication with Facebook, Google,

I would have to agree with xux's points. You really do want to make sign-up as simple as possible. Possibly even consider authentication with Facebook, Google, Twitter? (When trying new layouts/features you should keep A/B testing in mind) Also, admittedly I haven't signed up so I don't know exactly how it works, but to build on what namenotrequired said, you really do what existing members to be the ones bringing in people. Perhaps you could make it a bit more 'personal', by allowing users to inside facebook friends, and possibly building private league boards where a group of friends can complete against themselves rather than with strangers. I think it's these sort of strategies that will help encourage users to invite their friends. Although it is important to make inviting new users as easy as possible, please don't over do it. There are many apps that force you to connect via facebook, or constantly badger their users to invite people. These tactics may be effective but personally I'm against these very aggressive acquisition tactics at the expense of your existing members' user experience.