Invite your spouse
On Home, the card in the center is the one thing to do next. On a phone that card is at the top. When a parent is on the list but has not joined, the card names them. The first time, the button says Invite. It shows the address the email will use. Change, under that address, fixes it before you send. If there is no address yet, the card asks for one.
They get an email with a link that lasts 7 days. If they already have an account for that address, the link says Log in to join. If they are new, sign-up opens with that email filled in and locked, and a note that the invite was sent there. Not you? Use a different email clears the invite so they can start their own family.
If the link expires, that card comes back. It says Invite expired. and the button is Resend. A new invite retires the older link. A live invite that went to the wrong address is on Family, under Pending invitations: update the email, then Resend.
If they are not on the list yet, add them as a parent from Family and send the invite from there.