I have to be honest here, I hate signing up to test a product more so for invite on beta. Over the years of building online application I found out that is the case for a lot of people. You may want to consider providing a demo kinda open interface for your app.
Howdy friends,<p>I've just launched a beta of a new application that adds extensions to FreshBooks. Please take a look if you're a FreshBooks user! New modules coming out shortly for budgets, improved reporting, and some other goodies.<p>Thanks!