You can invite customers to use your store while it is invite-only.
To invite people, go to the Customers listing and click Invite New Customer. Enter an email address to add the person as a store customer.
There is a checkbox to Send an invite email, uncheck this if you do not wish the person to be immediately notified.
The recipient will receive an email inviting them to use the store. After clicking through from the email to set a password and accept the terms and conditions, they will have access to the store-front.
The email message will read:
Store Name Invite
A Store Name store account has been created for you. Please activate your account.