I don't think there are any actual requirements either way. I think that if Chris thinks you can review, and you ask, he'll let you. Similarly, if you create a part and submit it to Chris, he'll likely make you an author. (I could be wrong about this, though, and it could be that things have changed over time.)
To the best of my knowledge, the only reason I am an author is because I submitted the high-res (48) version of the eighth-sphere primitive, which I generated using the calculated coordinates from LDView's primitive substitution. (I later submitted a replacement for the standard-res eighth sphere primitive generated in the same way.) But I was reviewing parts before that, IIRC.
To the best of my knowledge, the only reason I am an author is because I submitted the high-res (48) version of the eighth-sphere primitive, which I generated using the calculated coordinates from LDView's primitive substitution. (I later submitted a replacement for the standard-res eighth sphere primitive generated in the same way.) But I was reviewing parts before that, IIRC.