The PartyBoost feature is not available when your JBL speaker is connected to a computer via Bluetooth. This is a technical limitation. When connected to a computer, the computer is acting as the primary device. Therefore, the JBL speaker does not have control of re-transmitting the Bluetooth signal to other JBL speakers.

NOTE: Speakers effected by this are the JBL Flip 5, JBL Flip 6, JBL Charge 5, JBL Pulse 4, JBL Xtreme 3, and JBL Boombox 2, as well as future products featuring the PartyBoost feature.