See video here.
Step 1:  On the host speaker (the speaker connected to your source device – Bluetooth or via the 3.5mm cable), tap the Stadium touch-button (top left button) to enable Stadium Mode (you will hear a series of tones).  Skip this step if Stadium is already enabled and other speakers are already connected.
Step 2:  On the additional speakers (and subwoofers), tap the Stadium touch-button to enable Stadium Mode. Rocksteady Stadium speakers and subwoofers that do not have a connected source device will enter satellite mode and will connect to the designated host speaker within 3-15 seconds (and will instantly auto-reconnect in subsequent sessions).