If you already accepted the invite to join a Premium for Family plan, but you're unable to see the Premium features, it's possible that you have more than one account and that you accessed the wrong one by mistake.
In this case, we'd like you to log out from the Spotify app. Then, log back into your account. If your acocunt was originally created with an email address, you'll need to use the username you chose and password to access your account (make sure not to use your email address). If your account is linked to your Facebook profile, make sure the "Log in with Facebook" blue button.
Let us know how if you're able to access your Premium account. Have a great day!