That depends on your exact setup. If you just want someone to be a moderator of a single forum (or maybe a couple) then add them as a forum moderator as Brogan suggests (you can add them twice for two different forums if required). However, if you want them to moderate several forums and only have a few that you don't want them to moderate, it's probably easier to make them a super moderator and then add them to a moderator usergroup which has all of the moderator permissions you want them to have. Then in the few forums you don't want them to moderate in, you can revoke these permissions.