The main requirement for being a tester? It's totally subjective. If you email Dave Ferrell, and ask nicely, he could let you into the testers group. However, it isn't very likely because there are plenty of testers. There are certainly enough that some were kicked out for getting on his bad side. There are probably more testers than actually test, thanks to holdovers from "the old days." It is my understanding some testers ask for and receive permission to let select non-testers have the test patches so they have some people to test with, but this is likely on a case-by-case basis, and not an open invitation to spam testers.
The folks who are in the group now by and large know the game very well and are good at testing for specific results or situations.
I'm only posting this to give my understanding of the way it works. I'm not a current tester, and I doubt there are any official policies on the matter of who is invited to test. There are also many SFC3 testers who were never SFC2 testers, and vice versa. The application to Activision for SFC3 testing has NOTHING to do with Taldren's semi-formal tester circle.