We have made a way to pre-setup share users for TOTP Two-Factor authentication. This removes the need to know the share user's phone number.
Go the Address book in the Sharing menu and find the receiver. Then press the "Invite to set up Two Factor Auth" button.
The receiver will then get an email that looks like the one below.
Steps for the share user
Click the link in the email and follow step 4-6 in Change From SMS 2FA Code to Authenticator App to set up the two factor authentication.