I wrote a long one and it was lost! =(
Here are some of the basic points I think:
Ignoring is a great idea. Don't have guests make a fuss. You don't make a fuss. It is natural for people to want to greet dogs when they first meet them, but for some dogs this is not okay. With a dog in my extended family, when I visit I ignore her for 5 minutes after I arrive, if not, she jumps all over me and can't contain her excitement. This also works for dogs that are way over the top when the owner comes home, just keeping everything low key and in some cases ignoring and not rewarding behavior when first coming in the door goes a long way. I believe in the book by Jan Fennel from the Uk (I don't agree with everything in there) she talks about this technique.
All the treats, extra attention, and focus on the dog when anyone comes to the door might not be good for some dogs, it might send the wrong impression. But then again it might work for him. Just depends, there are so many ways for different dogs to train them.
Some people have a dog bed or mat that is the dog's place. When guests come over, the dog is trained to go to place and stay there until released. Then after the guests are in and the dog has had time to settle, then introductions can take place in a low key way, with the dog approaching them if the dog feels like it. Or if the dog is super friendly, letting people go up to the dog. The message is you are the one that greets visitors and is in charge of who is welcomed into your home.
Def. keep up with the classes. He is young so you need to keep socializing the heck outta him. Take him everywhere you can and take yummy special treats for people to feed him if he is interested in meeting.
If you see he is getting stressed out, then just let him watch the world go by. Take him to get your oil changed, to kids games if you have kids that play sports, to drive-thru's, the bank, Lowe's and Home Depot sometimes in some places allow dogs, family get-togethers, friend's homes, the lake, the park, playgrounds, just everywhere you think you can. The more work you do now as he is a puppy, the better. Try for at least one new place a day. It will pay off later.
Don't let other dogs approach him in class, it isn't play time. Besides just saying hi is not recommended. http://www.flyingdogpress.com/sayhi.html
You can also use a good doggie daycare to help with socialization with other dogs. Make sure they are good though and know how to handle the dogs well.
With my wild child (not fearful, just over exuberant) I had to leash him when visitors came over and train him. I told him to sit (loose leash) made him sit or down (I wouldn't allow him to spin in circles and break his stays) and then invited visitors inside while they didn't touch him (he would get WAY too excited at first). Since he was on leash I had complete control over his behavior. After awhile since Dobes are so smart, he automatically did a sit when someone came to the door - regardless of leash or collar.
You can let them bark - but they have to be able to turn it off when you say so.
Good luck! Is there are class that specifically geared towards the CGC? That would probably help quite a bit with your specific training goals of wanting a CGC.