If “key” means a security key for a Microsoft account, it is added to that person’s own Microsoft account, not directly to the family group.
To add a security key for a family member:
- Sign in to that family member’s Microsoft account at Microsoft account page.
- Select Security > More security options.
- Select Add a new way to sign in or verify.
- Select Use a security key.
- Choose the key type: USB or NFC, then select Next.
- Insert or tap the key when prompted.
- Create a PIN, or enter the existing PIN for that key.
- Complete the key’s confirmation action, such as touching its button or contact area.
- Give the key a name so it can be identified later.
If the goal is to add the family member to the Microsoft family group instead, use a family organizer account:
- Sign in at family.microsoft.com.
- Select Add a family member.
- Choose Member or Organizer.
- Enter the family member’s phone number or email address.
- Send the invitation and have them accept it from their Microsoft account.
On Windows, a family member can also be added from Settings > Accounts > Family.