how to get an extra account for the subscription holder?

[happypost]

I have a Family subscription, and therefore 5 family members how can join my account. 

My own account is linked to the Sonos system in house. But when I go out, and want to listen to Spotify, the Sonos system is locked and vice versa.

So, I thougt, I invite myself again, on another email adres, in my own Family subscription. But when I do that, I get an error message, saying 'that I cannot added to my subscription'. How come? The emailadress is not the same. Does my device (iPhone) recognize this? How can I fix this?

Thank you!