Once you have music playing to a speaker or a group you can add more speakers from the "Connect" screen. Tap the orange Play-Fi logo in the bottom right corner to navigate to the "Connect" page. You will see the "zone" labeled in dark gray and right below, the track info and the volume controls for the speaker(s) currently playing.
Just above the speakers on the right, is a +/- button. This button allows you to add additional speakers to an already playing zone.
When you tap it, the Add/Remove screen will appear allowing you to check off the additional speakers you want to add.
NOTE: The primary speaker will show an "X" next to it. Clicking the "X" will bring up a warning message telling you about the primary speaker.
NOTE: Be careful not to tap the "+" button in the top right corner of the screen. This button creates different zones where different music will be played.