Sonos speakers can be part of a single Sonos ‘system’ at a time. You could add your friend’s WiFi as another WiFi source, but you can’t swap the speakers (well, easily) between two Sonos systems. 

If you wanted (and I am not recommending this), you could factory reset the speakers every time, and set them up as ‘new’ on the other Sonos system, but there’s not only effort involved, but you’d also be wiping out any saved data (streaming sources, playlists, WiFi information) on the speakers. 

If it were me, I’d just add the second WiFi, and use your own controller…but you wouldn’t be able to see or group any of your friend’s speakers. 

OK so what I really want to do is use two Move2 speakers in my car. I thought I could do it initially through bluetooth but found out stereo pairing doesn't work over bluetooth. 

Now I want to try to set up a wifi router in my car and run the 2 speaker from it. How do I set up a new WIFI System in my car with the SONOS 2 App? Once that is done I should be able to add my speaker to the new WIFI correct?

First, run a long mains power cable from house to car and plug in the router…

haha, I have a hotspot for the car. I don't need a long cable from the house

;)

Just follow the normal instructions for adding another WiFi SSID/Password to your Moves…or even better, set up the hotspot with exactly the same SSID and password your home network has, and the Moves won’t know it’s different (well, kinda, but they’ll just connect). Note that the controller (the app) needs to be on another device that connects to the hotspot, it cannot be on the hotspot itself. I think. The old app required that, I’m not sure about the new one, but I’d be stunned if it was different.