The signup tells you to sign up for something called TimeCamp[0], which appears to be a paid time-tracking SaaS. GetInvoicing probably sets you up with a TimeCamp account, and allows them to market to you.
If you're interested in a really free (as in no registration) invoicing tool, http://invoiceto.me/ works a treat for me. Been using it for months now.
Hi, thanks for comments. In fact invoicing will be free. But we want to build another set of features around this like powerfull time tracking, and charging that. So it will be like freemium model.
I'm guessing they are planning on using this as a lead gen for other projects. Logic being; free product gets popular, developer gets other gigs which are paid.