When creating contacts it is normally preferred for them to be created with their company already assigned to them. So, whether it is an existing contact or you are creating a new contact, you can always assign it to an existing Account.
If you are creating a new contact, simply go to the section labelled 'No Account', click the search button and enter the company assigned to the contact.
If it is an existing contact and you want to assign an account, it only needs to be edited. First click on the magnifying glass icon below your name to search for the account to which you want to assign your contact.
In the following video we see how to assign a contact, the account had already been created: