Up to now, you could only assign a contact to a company by editing the contact and setting the Company field.
You can now add contacts directly to a Company.
On your mobile, goto the Edit screen of the Company and under the Linked Items section, select Contacts. You can either add an already existing contact or create a new contact.
In your web account, click on the New button from the View Company screen, and select New contact. You can either add an existing contact from the drop-down list or create a new contact.