The Playbase has no idea whether your TV is on or off. Please describe exactly what your problem is.

Unless for some reason, the power for the PLAYBASE at the mains is being turned off when you turn off your TV set, you still should be able to ‘group’ the PLAYBASE with any other room that is streaming music. There is no ‘power’ button on a PLAYBASE, it just goes into a low power state when not being used. 

There is a TV Autoplay feature, but not an Autogroup to connect it to other speakers/rooms.