I was on a family plan and it kicked the two of us who were not primary account holders off. The account holder has tried re-inviting us, but when we click on the invitation link, it does not accept our address. It worked a few months ago when we joined, then we got the email asking to confirm our zip code, we entered it, but then got an email saying it was incorrect and that we would be switched to the free account. I don't know what the issue is, as we are all entering our address correctly.
Since you have already tried to re-invite, I recommend reaching out to Spotify Support. They can be reached through their Twitter Handle @SpotifyCares, Facebook Page Spotify Cares, or through the online contact form.
If you submit the contact form instead of the Twitter or Facebook Message, make sure to check your inbox and junk folder for an email from Spotify.