Surely a streamer which supports Spotify Connect is what you want? That way it's streaming directly from the Internet to the streamer unit, not via an intermediary playback device (i.e. phone/tablet which then has to do AirPlay; chewing up battery and hogging more bandwidth on the network). Any of the RPi-based streamer units should suit - I use the Allo DigiOne but there are plenty of options.
As long as all devices are on the same network, family members can simply select the streamer as the playback device from their Spotify app on their phones. We do this with the streamer log