To do this, log in to your account. Then go to the Businesses tab and click Add First Business. On the pop-up, type in the first and last name of your client, look for the business on the Business name and main location field, enter the email address, and if you want to create the password for them, you can tick the Create Login check box and type in the password. After that, hit Save.
If you did not tick the Create Login checkbox, they can create their own password by going to your domain and clicking Forgot Password and entering their email address (the one you used to register the business).