The science within the mission of the NIDDK is extremely broad, and consequently we are looking for reviewers with a wide range of scientific expertise.
Most of the individuals that review for the NIH receive or have received NIH funding in the past. While experienced reviewers are always welcome, we also actively recruit reviewers that are new to the NIH. Holding your own NIH grant is a plus, but it is not a requirement.
Although senior scientists will make up the majority of a review panel, scientists in the early stages of their career are also welcome at the NIDDK. While you won't be asked to review if you're a graduate student, we will certainly consider other junior scientists. If you are in the last years of a K award, consider serving as a reviewer to gain insight before you submit your first R01.