Have you tried checking junk folder and see if your invitation is hiding there? I'd also recommend making sure that the invitation was sent to the email address associated with his Spotify account. Let me how it goes 🙂
Hi, I cancelled my subscription before the renewal date and I can still play my songs offline even if I am no longer subscribed. I am not seeing I was charged though, according to the help forum, I will lose access to my offline music once the subscription expires, I am confused.