[orky56]

1) Create a landing page with an easy-to-type and remember URL.

2) Convey the basic value of the app (why it matters, why it's relevant, why should they use it over the competition, etc.).

3) Give users ability to give email address and other optional info such as name or other survey questions.

4) Create at a minimum a chance to have users give their friends email addresses. Even better would be to give them an incentive to invite (quicker access, additional features, etc.). Check out untorch.com for a easy turnkey solution

Using a mail service, you can automate sending an email with the build invite.