Hi Dawn! We're glad that you are enjoying Tetris Battle.
You are able to challenge your friends in Battle 2P, Battle 6P, or Sprint 4P. Look on 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.
Click on a friend that has a “JOIN ME” label and an invitation will be sent for your friend. 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.
More details instructions can be found in the Help section in the General tab.