How do I make one of the members of a YahooGroup into a moderator for that group?

Go to "Members", then click on the member that you want to change to Moderator. Below their member name, click on "Edit Membership". In the lower part of the box, click on "Change to Moderator". Then there are several check boxes with different privileges listed. You can check whatever privileges you want to allow for that member. There can be many moderators for a group.