If you are the company admin you can access the company settings. You can do this by going to the menu in the top right corner of the web interface. Click ‘Company settings’.
In the ‘Account’ tab you will find ‘Company domains’, at the bottom of the page. Users with email addresses belonging to any of these domains will be listed under ‘Co-workers’. Enter a domain you want to list as one of your company domains and click ‘Add’.
Note: in order to add a company domain to your company account, you will need to have an email address belonging to that domain linked to your personal account.