Inviting a family member

Admins invite relatives one at a time with a personal invite link.

Open Family members (website: avatar menu → Family members; app: Me tab → Members & invites).
Tap Invite, enter their name, a username, and their role (usually Contributor).
Optionally link them to their own person in the tree, and email the invite.
The invite link is copied for you — send it however you like.

When they open the link and set a password, their account and their own tree are created automatically.