If you're referring to the Spotify Family plan - there is not currently a way to link the individual Spotify profiles to Alexa - you would need to register the Echo's to each person's separate account.
However, Spotify Premium on Alexa supports Spotify Connect, allowing you to play music on your Alexa device, using the Spotify app on your phone or tablet as a remote. Using this method, you could keep all your Echo devices registered to the same account. Learn more about Spotify Connect here.
Below is copied and pasted from one of my other posts...
Using the household system you can add ONE adult account the household. https://www.amazon.co.uk/myh/households You can add child accounts (age up to 12yrs) but as far as I can tell it has no effect on the way echos work i.e. child accounts can't be used on echos. The additional adult account can see all the echo devices and allows you to share some other amazon services. Under the T&Cs 13 is the minimum age limit to use Amazon Music Unlimited Family which in turns needs an Amazon Account....
As far as music goes, in a family/home environments depending on your requirements it can get complicated. I have been playing around with Spotify Premium Family and Amazon Music Unlimited Family, this is what I have worked out so far for the UK, the USA offering seems to be more advanced than ours..
Echos on separate Accounts: will mean that you can all have your own music accounts ((Amazon Music Unlimited or Spotify Premium)&(individual or family plans)) and any personalised recommendations will be personal to you. The downside is that you loose the "announce" feature and your ability to listen to your music in your account is restricted to specific echos (the Echos registered to your account). And I suspect that a host of other features like shared shopping lists, shared calendars and control of electrical equipment in "smart home ecosystems" will be adversely impacted.
Echos on the same account: will mean that you have your music on a shared account. With Spotify Premium (incl family plan) you can only listen to music on a single Echo, so if somebody else in your house tries to listen to music on a second Echo then the music will stop playing on the 1st. You also won't have personalised recommendations (Daily Mix) because you are listening to music on a shared account. If you use Amazon Music Unlimited Family Plan it is still a shared account so no personalised recommendations but you can stream to more than one device simultaneously (upto six). As a workaround you could stream to a phone or tablet and the use the Echo as a bluetooth speaker, that way you could still keep your personalised recommendations.
There is a thing called Alexa Voice Profiles, to create them you need to logout of the Alexa App (Menu...Settings...scroll down and logout). When you login you will be given the option to create voice profiles. In the US, voice profiles used with Amazon Music Unlimited Family Plan allows Alexa to customize music playback based on voice. The idea is that your voice profile is linked to the amazon music account. Unfortunately it does not appear to be available in the UK and/or on the trial version. The US version looks something like this https://www.amazon.com/b?ie=UTF8&node=17368907011 . For the UK there is no real way to manage these voice profiles e.g. deleting, linking voice profiles to amazon music accounts. I can't see how you link anything to the voice profiles, and I doubt it will support Spotify in the immediate future.