There are two steps to creating a user. First we'll run the smbpasswd utility to create a samba password for the user.
sudo smbpasswd -a username
Next, we'll add that username to the smbusers file.
sudo gedit /etc/samba/smbusers
Add in the following line, substituting the username with the one you want to give access to. The format is = "". You can use a different samba user name to map to an ubuntu account, but that's not really necessary right now.
username = "username"
Now you can create samba shares and give access to the users that you listed here