I've moved your question to a new thread because it's a different issue to the other thread, dealing with 'guests' vs 'clients'.
Your sheltie is showing typical farm dog behaviour - my boy does exactly the same thing. He also sounds like a sheltie with a strong herding instinct. Will he grow out of it - nope, he's rehearsing the behaviour he is likely to continue.
Guests arrive and he is alerting you of this. It isn't aggression or annoyance at the visitors, it's over excitement. Once the visitors cross the threshold they are part of the flock - so being a sheepdog he gets distressed when one of the flock leave.
The best way to deal with this is treats - but they must come from you, not the visitors. Having the visitors give him treats make the visitor even more exciting. You want him to focus on you, not them.
Firstly, identify his 'triggers'. Does he go off when the doorbell rings, when he hears the gate or a car, when the door opens? Usually it is a sound that starts them. When people leave do you go through the same routine - use specific words to say goodbye to visitors?
Secondly, put a sign on your door "Sheepdog in training" so people will be a bit more understanding.
With people arriving you need to disassociate the trigger with the event. So if the doorbell or knocking starts him off then keep pushing the doorbell/knocking at random times, if it's the door, then open and shut the door. Do this lots so he doesn't assume a bell/knock etc always equals a visitor.
When a visitor arrives you want him to alert you but not bark at the visitors. Start with putting him on a lead and keeping high value treats handy. Get him to sit before you open the door and look at you - treat him whenever he is calm and quiet. Let your visitor in and lead him away from them and the door (ask them to shut the door for you), keep him focused on you with treats. When you are settled and he's not as excited release him to he can say hello to your visitor - your visitors should never greet him if he isn't calm.
A similar process is used for visitors leaving. Avoid long farewells and trigger words. The phrase "see you" is now banned from my house (it's been hard). To start with ask your guests if it's okay for them to make their own way outside, you stay seated with sheltie, get him to look at you and give him treats for being quiet. When he's doing this okay you can start taking him with you as you escort your guests out, but keep a distance between you and the guest, keep the sheltie on lead and keep him focused on you with treats, initially let your guest open the door for themselves. And remember - no trigger words. If you are seeing a car off, turn around and take your sheltie back to the hosue before the car takes off - the excitement of a moving car will undo all the work.
As you work on this process the outcome should be, a visitor comes, your sheltie may bark a little but will come to you, the sheltie waits for your visitor to settle to say hello. And when they leave your sheltie will stay by your side.
It does work long term, but as you are working against instinct I've found it is necessary to constantly reinforce the behaviour. I keep treats next to the front door, Deska does always get a treat for the correct behaviour, even tho it's just the tiniest 1/4 of a yoghurt drop it's part of the routine. And I just can't say the trigger words to guests. Instead I use whole sentences instead to say goodbye.
