you could look at
airfoil.
it lets you send audio from any one program (or whole system audio) on your computer using airplay.
there are separate licenses for windows and Mac though.
This does have the advantage of not sending notification sounds to the shared speakers.
then get an airplay speaker, or multiple, and you can stream to them.
I'm a huge fan of Sonos, the One (not Play:1) will let you airplay to them.
if you add other non-airplay Sonos, you can stream airplay audio from the one with airplay to the ones without.
Or, even better...
Sonos has built in players for most of the popular streaming services, as well as tune-in radio which has a lot of local radio stations.
It also allows multiple accounts for each service,
more info
so everyone can login to their favorite service, and then play directly on the speakers,
once music is selected your phone/computer isn't needed.
Once Sonos is up and running, you just have to launch an app on the network to control them.
so Bob would be able to see and even pick from Mary's account If he wanted.
or if you wanted, you could add....
an album from Bob's Spotify
an album from Mary's Apple Music.
a couple songs from Greg's Google music.
an album from Dan's Deezer.
an album from Steve's Spotify.
These could be added to the end of the queue by each person, or one person could queue the whole list.
and it would just play though the list like a juke box.