Friends can be added to Pool Blitz by linking Facebook to your account. Once linked, any Facebook friends who also play Pool Blitz will appear on your friend's menu.
You can connect your account to Facebook via the in-game settings menu (grey cog icon), by choosing the "Secure" option under "Profile Management".
Once your account is connected, you will be able to challenge your friends to a match of 8-ball or Blitz! To do this, head to the friend's menu (bottom right corner of the main screen). Facebook friends who also play Pool Blitz will be displayed here.