How to add user to Ubuntu Phone?

Normally I would just do:

sudo adduser anewuser

and then provide the initial password.

However in Ubuntu Phone I get:

passwd: Authentication token manipulation error
passwd: password unchanged

What can I do?

Error passwd: Authentication token manipulation error occurs generally in following scenario:

• File system mounted as read-only.
• Incorrect Permissions on system files.

In your case, as you're working with Ubuntu Touch, I'm guessing that your system is mounted in Read-Only mode, as this is general security feature of any Mobile Phone OS to reduce risk of overwriting System Files by any other services/application accidentally or purposefully.

Solution could be remount file-system in Read/Write mode, and also by checking File-permissions & Correct it if required to make sure you're changes will make through!

• Remount File-System
• Check File-permissions and Remount File-System

Ubuntu Touch currently is not designed for a multi-user use case.