When to build links to a website in construction?

I'm building a new website and only marketing on Twitter right now. The reason for doing so is because the only page that the website has is a splash page describing the site and has an e-mail sign-up system.

That one page is indexed in Google.

The planned progression of the site is:
Splash page -> invite-only beta (still one page accessible for non-members) -> open site

So when would be the ideal time to start building links? Thanks!