Each new moderator should recruit one or more people willing to serve as a backup, on a permanent or temporary basis as needed. These backups should be located as soon as possible after the moderator is selected.
The need for a backup moderator depends a lot on the nature and volume of the group. A newsgroup that contains mostly pre-approved FAQs from other groups, such as some of the *.info groups gaining popularity, needs backup moderators a lot less than a high-volume discussion group or a time-sensitive *.announce group.
Having others who can fill in temporarily, if the need arises, serves as an insurance policy for the primary moderator. You may need to take some time off from moderator responsibilities due to work schedules, vacations, or net connectivity problems, to name a few common reasons. In those cases, having a pre-selected backup assures the newsgroup's continuity during periods when you are unavailable.