You can manually add and maintain your customers from the Customers menu. Additionally, the Customers themselvs would appear in the listings by using the online store registration form.

Manually Adding a Customer

In your admin panel > go to Custmers > Add New Customer in the top right corner.

Once you open the menu, you would need to fill all boxes with the required information:

Customer First Name
Customer Last Name

The following options are available for your Customer:

Customer Note - You can write an additional information/comment for a customer, which will be visible only for you.
Customer Group - If you had not created a Customer Group yet, you should set the option to Default.

If you have any previously-created Custom fields, they will appear below the Customer note as an additional choice.