You can create a new user from your customer account. Go to Settingsunder the User accountsection in your customer account overview. Then click on the Add new user button, fill in the details and click Save.
If you input a username when you set up the account, the account will be associated with a specific user. This user will be able to log in with and amend the data registered with the customer account.
If you don't input a username, the newly created user will be registered as a person. This person can place orders by phone or e-mail and go and collected these orders. However, they won't be able to log into the customer account.