Limiting Site Access

I am looking to do a "soft opening" on an ecommerce site to prove my product's viability on a small scale.  Is it possible to have an ecommerce store and to limit access to my site exclusively to those that I give some sort of permission?  In other words, a customer contacts me directly, and I give them the key to my site.  Is this possible?

Any advice will be greatly appreciated.  Thank you!