Account owners, account managers and billing managers can add users from Control Panel -> Users. Each user signs in to the Web Portal with their own username and password, which is emailed to them when they are added.
A user's role controls what they can do in the Web Portal:
Read-only users are free. A user who can edit data (including importing sessions) uses a license, either one of their own or one shared with a device. When you add or edit a user, choose one of:
Linking an existing device keeps its Unit ID. A new device created this way gets the user's username as its Unit ID, so the user enters their username as the Unit ID in the app. Account owners use a license. The primary account owner can instead be given Billing Manager access (Users list -> Make Billing Manager); they stay the account's billing contact and do not use a license.
The Users list and each user's page show their license and linked device. The license allocation table on the Account page shows every license and how many are free; users without a device show a "Link a device" link there.