Answer:
'smbpasswd' is the program you are looking for. Read the manual 'man smbpasswd' for more information. You must be root or run with sudo to change other users passwords.
Change password with the command 'smbpasswd <username>'
# smbpasswd bob
or
$ sudo smbpasswd bob
NOTE: You may also need to enable the user after you set the password
# smbpasswd -e bob
or
$ sudo smbpasswd -e bob