Anyone with Manager or Admin access can add users. There is no additional cost or limit on adding users to your account.
First, navigate to Settings > Users.
Enter the user's name and email address, and select a level of access.
User: Users can only connect their own inbox in Settings, and can only see the replies in the inbox that pertain to their connected inbox. They do not have access to skill, or other permissions to view or edit.
Manager: Managers have access to create, edit, and manage across all functions in the account (including users and integrations). They cannot access the Subscription tab in Settings to view billing information.
Admin: Full access to all functions in the account.