You can play with friends in Battle 2P, Battle 6P, and Sprint 4P as long as they are:
1. On your Facebook Friends list.
2. Currently playing in Tetris Battle (https://apps.facebook.com/tetris_battle/).
3. Not playing with another friend in Tetris Battle
To play with your friends in Battle 2P, Battle 6P, or Sprint 4P, look for the Friends Bar (on the bottom of the game) for a friend with the label “JOIN ME”.
That label will tell you your friend is online and available to play with you.
You can also click on the refresh button in the Friends Bar for immediate status of who is available to play.
When you click on a friend that has a “JOIN ME” label, an invitation will be sent for your friend to respond to.
He or she can join in or cancel the invitation.
After he or she accepts you will see a chat box on the Friends Bar. When you are ready to play, click on start and enjoy.
If you are playing Battle 6P or Sprint 4P, you can add more friends into a game, one at a time, by clicking on another friend with the “JOIN ME!” label.
Play Tetris Battle ~> http://bit.ly/IBBEaD
If you are unable to get this feature working check out this page: