ACP > Members > Staff > Moderators
Check to see if you have an existing member group for moderators. If not, add one. You can edit it to select which specific permissions they should have (and where). If you don't apply any limits, they'll be considered to be unrestricted moderators.
Once that is done, assign your specific moderators to that group. So if "Bob" is supposed to be a moderator, set it as his primary or secondary member group.
ACP > Members > Members
You could just assign moderators without setting a group, but it means you have to set permissions for each specific person instead of the group and have them inherit it automatically.
Also take a look at the following guide for setting up moderators: