You must be invited by a contributor to join a journal. The contributor can share with you an invitation key (by email, SMS, WhatsApp, Messenger, Instagram, etc.) that you'll have to copy and paste in the "Join a journal" section of the Neveo app.
This key should only be used once, at connection, to allow you to join your family's journal. Afterwards, you will just have to log in with your credentials, without using the key again, to access the shared journal.
For further information on how to invite a contributor, please visit the “Add a contributor to a journal” section.