I am on OS X Mavericks, and I am admin. I need to reset another user's password. So I choose:
System Preferences -> Users & Groups and clicked the lock icon to make changes (and entered the correct admin password)
But the other account is still greyed-out, so I cannot choose it to click Reset Password for it.
Not all accounts are grayed-out, but at least one is.
If the other account is logged in (possible if you have "fast user switching" enabled), you will not be able to make any changes to that account from System Preferences, even as administrator. To reset their password, they cannot be logged in. If you can't log that user out, you can restart the computer to force it to be logged out.
If you have fast user switching enabled try disabling it from the login items then restart. That resolved the problem for me.
