If you want to create a new customer under a main company, make sure that you add the customer while you stand in the main company.
Here's how you do it:
Choose the main company in the upper right corner.
Touch the menu in the upper right corner
CVR: Enter new customer's CVR no.
Company name: Enter the new customer's company name
Contact first name: Enter new customer first name here
Contact last name: Enter the new customer's last name
Contact person's email: Enter the new Customer's email address
Create Customer: The button needs to be activated to update the page
When the above is completed and you have clicked on "Create customer", you can find the customer by clicking in the menu in the upper right corner and selecting the customer from the list.