I too am having this problem. I JUST signed up for Premium Family because we are leaving on a trip tomorrow. My daughter wants to keep her playlist so I am trying to invite her using her same email account. But I get an error every time I invite her. I'm super frustrated because that is the ENTIRE reason I signed up for Premium Family.
Hope they resolve this in the next four hours or I will have to cancel.
I just don´t understand how it works with this sending a link thing: who will know I send a link ? And how does that person get the link ? There´s no mail or number involved; so I just don´t understand how it connects to the wanted one?
Sending a link via email or SMS didn't work for me either. Some random error about the user already existing, when they don't, then an error with unsupported browser. Swapped browsers and devices to no avail.