If you WordPress site is running a WooCommerce store, there will be an area for your customer to create an account and then subsequently login to their account when using your store again.
This page will normally be found at https://www.yourdomainname.co.uk/my-account (unless this has been customised for any reason)
Registering Your Customers Account
Direct your customers to the correct page on your site above and they should see something resembeling the image below;
The customer simply needs to enter their email address and a password into the register section and their account will be created and an email sent to them to confirm this action.
Once this has taken place this user can then be found in your wordpress user area as a customer. You can see more information about the standard user role types here.