Can post-docs serve as reviewers for grant proposals? I am always hearing that being a grant reviewer is  a great experience in learning how to write proposals. I am a post-doc with an eye on an academic career so would like to do this. Does it happen? How do I sign up? I am in physics and in USA, if that matters.
Absolutely a postdoc can be a reviewer, if a program manager thinks that they would be a good reviewer to have.  It's more rare than having more established people, but there is no prohibition.  In order to increase your changes of being invited, you need to get to know program managers and let then know you are interested in serving for reviewing.