From the notification menu > select 'Company Admin' option.
You will be redirected to 'Company Admin' page.
Click ADD USER button. This opens up feature to add a new user to the company.
Enter data. For 'Role' pick option field, select a role based on how much access the user need to have.
Admin = All access.
Standard User = non-admin role. Restricted from some pages. Can only view own orders.
Standard User with Data Visibility = non-admin role. Restricted from some pages. Can view all seller's orders.
After data is entered, click SEND ACCOUNT INVITE and the user will receive an email that provides functionality in the form of a button to set up an account on the domain. Once that is set up, they can access the domain as a full fledged seller.