Work independently, but pair up to reach agreement by email. Then report back to the main list with findings, defect fixes, etc. If code reviewers do not report back, then other code reviewers can be recruited or assigned. The open market of the bazaar model which is successful in generating enhancements is not going to be as effective at motivating code reviewers. It is not necessary to have the same code reviewers for successive reviews.
Code reviewers can come and go.
Concentrating on only the changed sections is most effective, but inspecting patches or context diffs is not always as effective as looking at full or unified sources. Encourage the use of the unit testers to verify all new code and defects.