Keep in mind that only one Spotify account can control the music with voice commands within the same Amazon account.
It's possible to add other Spotify accounts to the same Amazon account, but these won't be able to use voice commands.
It is however necessary to have your Spotify account connected to the Alexa app in order for the speaker to appear in the list of available devices on Spotify. It will not appear if your account is not configured.
If several users want to use voice commands on the same Alexa-connected speaker (e.g. users in a Premium for Family plan), they can try the following:
Download the Alexa app on their own device and log in with their own Amazon account (see above).
Connect their Spotify account to the Alexa app (see above).
Now, every time a new user wants to use voice commands with Alexa, they need to say "Alexa, Spotify Connect", and then choose the Alexa device from the list of available devices in the Spotify app.
Amazon Echo, Amazon Echo Dot, Facebook Portal / Portal+, and all Sonos players.