I saw something like this on tv a while back.

That dog had been brought up in an office, too, & the owner called this pet show for help.

turned out the dog was protecting the territory & protecting his owner.

The owner had to take over the job of alpha dog, putting the dog into a subservient position.

I wish I could remember how it was done, but maybe someone here can provide some pointers.

About that waiting until the people are on their way out:
it's the same thing that makes dogs bite postmen.

The dog is guarding his domain, & the postman is an interloper.

The dog watches & guards & sends ESP or some such, & when the interloper turns & leaves...

"It worked!

Guess I showed him!

I'll just finish chasing him off!"

I wish you the best.

Is there a way to keep him in a back office or put up a barrier so he can't reach a customer? I wouldn't take a chance with this at all if it were my dog. The consequences for you, him, your business, and the victim are too serious if he were to bite someone.

You'll still want to work on training and getting him under control, but do it without endangering anyone.

What is the breed of the dog, and how much exercise does he get? I ask what breed he is because I'm wondering if he's got some herding instinct in him.
Whether he's a herding dog or not, it sounds like he needs basic obedience training. The first thing you need to do is make him stay off the sofa since it has become his kingdom, especially since it's next to your desk.
The second thing to do is enroll in an obedience class if he's never been thru one.

In the meantime, keep a leash on him and make him stay on a dog bed near or under your desk. This will allow you to keep control of him while you work on corrections.
Things WILL get worse if you don't nip it in the bud now.

I agree with others that you need to take this young fellow to a class. Look for one that uses "clickers" and that uses positive training methods. I agree with Sylvia that your pup has learned that when people stand up, they are going to leave. This means that if he chases them out, he always "wins." This makes him feel successful. That is why is it so hard to get dogs to stop barking at mail carriers or delivery people. Dog barks, person leaves, dog wins! A positive trainer will help you break this behavior now while he is young, but a positive basic training class to get you and the pup accustomed to using clicker training will be needed first. Clicker or positive training is better because it engages the dog in figuring out what behavior will get him rewarded. It builds a great relationship between you and the dog and teaches both of you tools that you can use to address any problems that might pop up throughout your dog's life.

Here is a link that might be useful: Info from the Association of Pet Dog Trainers