Well CT you will learn that there are many ways to contribute. Reviews is certainly one of them but not the only one. There are lots of back channel communications that some are privy to and some are not. Likewise, some are privy to a few communications but not others.
Most everyone I know here, at least as far as the guys, was by invitation to some back channel group. Why did I get invited? Hell, I don't know. I am also aware of other groups that I have not been invited to.
So, don't assume that just because you can't see someone review, that they are not participating or contributing to the community. They just may be contributing at a level that you don't see.