If you want to test just one app, you should be able to do that, without any hassle, and be a tester. If tester here means something else (as in a tester who is a more devoted tester than an ordinary tester), it should be made clear. We don't want to scare people away with special procedures or by making them think they should do something special to be a 'tester' (i.e. not to loose people to nomenclature, like seen from RevdKathy's post).
And this is my main beef. The testing squad should certainly help stuck apps, but should also stay clear of replacing users. If the person in charge of an app does not understand what an app is supposed to do (or interested in it), it's not realistic to expect a good QA process. Organized testings also have the problem that the different testing squad members may be in different time zones, different schedules, etc (for example most of my free time is weekends - that's more than 5 days, and I will hardly be able to do anything during workdays so anything that should be done by Friday is an automatic fail for me).
Stuff that is missed out from my QAmaster proposal altogether (just mentioning it so we are in the clear if this is intentional): the cooperation of tester admins with actual developers. Testing, if issues found, should ALWAYS result in a bug report(s) and, preferably a pointer for the developer how to resolve it. Remember, the goal is not to clean out extras-testing (that's just a consequence), but to help streamline promotion to Extras with a special focus on quality.