Two options will appear. You can either add an existing Pressbooks user or someone who does not have an account yet.
To add an existing Pressbooks user:
Fill out the Add Existing User form. Enter their email and assign them a role using the dropdown. Then, click Add User.
To add a user who is not already using Pressbooks:
Scroll down to the Add New User section. Give the user a username and role, and enter their email in the box provided. Then, click Add New User.
In both instances, the user wil be sent an invite email to your book.
For more information on how to manage your users and collaborators, check out https://guide.pressbooks.com/chapter/users/