To join an existing Tribu subscription, you must be invited by another member of that Tribu. Each member can invite others by clicking on "invite members" in the menu. He or she can then choose to:
send an email to the person he or she wishes to invite
copy a link to share (on your WhatsApp family group chat for example)

As soon as you click on this link, you will be prompted to create an account (if this is your first visit) and you will be added to the Tribu subscription automatically.