Yes. They can use Spotify Connect when they’re on the same wifi network as your Sonos. Be warned though - it is possible they can still play via your devices when they’ve left. It is probably better to have them text you their playlists.

So, even when not on my Wi-Fi they'll be able to play Spotify on my devices using Connect? 

I’ve read that this is possible. I’ve no personal experience with it.

I think they have to be on your Wi-Fi in order to see your speakers so that they can then use Spotify Connect to ‘cast’ to them. I don’t think your speakers can be ‘seen’ to initiate the first stream without  being once on your network. Once they’ve been ‘seen’, the Spotify app would store credentials until the connection was broken by you.

I think. 

Right, I mean to ask: once they've connected to my speakers on my Wi-Fi, they'll always be able to play music on my system via Spotify no matter where they are? 

Only until you break that connection. I think that’s done by you playing something other than Spotify on your speakers. It’s not an infinite connection. 

Ah. That sounds plausible. As I said I’ve read about this, but have mot experienced it myself.

Same here ;)

Bruce is righr on all counts here, I believe. The Spotify app connects to a speaker that is Spotify Connect compatible and happens to have a Sonos badge on it. But it could be any Spotify Connect compatible speaker.

The Spotify app discovers the speaker over the LAN, but the connection is made in the cloud. So it survives even after the controller disconnects from the network.

Playing another source does, as Bruce says, break the connection.

I have done this, purely as a scientific experiment!

Thank you for the confirmation, sir!