I'm using hifiberryOS on a pi3 with the 7" touchscreen and hifiberry amp, works a treat. We mainly use internet radio and Spotify in the kitchen via a couple of Cambridge Audio speakers, sounds lovely.
We have a family premium account, each family member effectively has their own account. So only if your account credentials are in the /etc/spotifyd.conf file can you use the service via your spotify app on your phone etc. As far as I know this only takes one user at a time.
This is a bit of a pain with a growing family who all have their own play lists.
Unless there are better ways, could there be multiple spotifyd.conf files and a means of selecting which one you want to use via the GUI? I believe account details are cached in spotifyd so this may need to be flushed between user account selection,
I guess this is an enhancement request unless the solution is already out there :)