LIMITED TO 5 ACCOUNTS ONLY
You first need a Spotify account located in the US. If it’s not the case, use a VPN located in the US then change the location into the US in your Spotify settings.
Then follow the invitation link and log into your account.
Use this invitation link https://www.spotify.com/us/family/join/invite/cAx13c0a7x8AA9C/
If they ask you a US address, use: 76 S Lander St, Seattle, WA 98134, USA
Voilà, 3 months of Spotify Premium for free. It’s repeatable.
As for 9/3/2020, confirmed working, Please try your luck before the invitation goes off.