Go to the Sign up to Once page, and enter the total number of users that you want to be able to use Once Mobile.

e.g. If you currently have a single user subscription and want to have 4 users you must enter 4 for the number of users.

For full details on adding/reducing the number of Once Mobile users, including the changes you need to make in Once Setup and how it effects your subscription, take a look at the article on our Support Portal.