Configuration

/etc/samba/smb.conf  You knew it. 
Only security and [homes] are not default, the others are default. 
In this configuration, all the \\server\\username can be accessed.

[global]
   workgroup = WORKGROUP
   server string = %h server (Samba, Ubuntu)
   dns proxy = no
   log file = /var/log/samba/log.%m
   max log size = 1000
   syslog = 0
   panic action = /usr/share/samba/panic-action %d
security = user
   encrypt passwords = true
   passdb backend = tdbsam
   obey pam restrictions = yes
   unix password sync = yes
   passwd program = /usr/bin/passwd %u
   passwd chat = *Enter\snew\s*\spassword:* %n\n *Retype\snew\s*\spassword:* %n\n *password\supdated\ssuccessfully* .
   pam password change = yes
   map to guest = bad user
[homes]
   comment = %U Home Directories
   browseable = Yes
   read only = no
   create mask = 0660
   directory mask = 0700
   valid users = %S

For other user(not the user created during the OS installation but created with user add)
We still need to sudo smbpasswd -a <username>

Could we use unix user dirctly?

Show smbpwd users

$ sudo pdbedit -Lw

Test samba

Linux command line

smbclient -L <IP>  -U guest -N #list
smbclient -L <IP> -U <username> #list

smbclient \\<IP>\<username> -U <username> # login

Mount for current user

sudo apt-get install cifs-utils

sudo mount.cifs -o username=<samba username>,uid=${USER} //<samba server>/directory/  /<local directory>

Windows

You may get 'Multiple connections' or 'disconnect previous connections' error in windows
rem list the connections
net use
rem remove all the connections
net use * /delete

usershare

Allow non-root/sudoers to create shares