With every new release, theres always new code. This code is tested by the relevant developers. But this is not the preferred situation as sometimes the developers are to close to the code and miss testing things that they should.
Whats requires is a person(s) who are willing to test the Simple Invoices code while its in development. Perferably to develop test cases and test plans and work with other testers and developments to iron out as many bugs as possible before any releases.
If this sounds good to you feel free to post on the forum