From the Bang & Olufsen app, music must be first playing on a lead speaker to expand this music to other Multiroom-enabled speakers. Any of your Multiroom-enabled can be the lead speaker, but only one at a time.
To play music from one speaker, select the speaker you want to be the lead speaker in the Home tab. Then, from the Now Playing window, or from the Music or Radio tabs, start music playing on that speaker.
Music is expanded to multiple speakers from the Multiroom screen. That can be accessed from the Multiroom tab, or from the Multiroom icon in the Now Playing window.
In this screen, the currently selected product will be shown at the top, and all available products in your account below.
Select the product with which you want to share the sound with the plus sign. The music will begin playing on the selected products.
Only speakers which are currently added to your account and are currently connected to the same Wi-Fi source will appear as available in the Multiroom screen.
You can also remove the Multiroom experience by unselecting products that have already been shared to.