Want to try and correct this

Well I think I am hitting the rebellious stage of puppyhood and coming across a few problems that I need advice on.
He goes to work with me and my partner everyday and when people come into the office he barks a little then runs away and if the person is invited in or he knows them he is fine with them... But sometimes cheeky people let themselves in and Otis doesn't like it he barks for a bit, runs away, growls etc. Today when a man let himself in he did the same behaviour and the man came over to stroke Otis he accepted it but as he went to carry on into the office he growled I was not impressed and gave a firm no! I think he's trying to alert us to an 'intruder' as they just take the liberty of letting themselves in... What can I do to improve this? Make sure one us invites the person in? Correct the behaviour with a firm no and sent to bed? Just don't want it to escalate and with him being my first ever dog I am on edge.

I think the key is to educate the people who come in! Take Otis to his bed (I take it he has somewhere to lay?), and give the visitor a couple of treats. Tell them to hold the treat in the tips of their fingers & just let their hand rest down by their side & don't look at Otis or talk to him. Let Oitis decide who is safe to check out & soon he'll realise that visitors mean treats!

Yeah he has a bed there but he tends to want to sit with me in the chair or run around with the bed lining in his mouth
Definitely couldn't agree more with educating visitors, some of them just don't think!! Will give this a whirl and update as soon as... I don't think it's anything nasty but today made me think right this needs to stop and the man who came in is a very tall, loud man which he was probably unsure of!

We have this problem in our office with the MD's dog - she will bark and growl at everyone she doesnt know so we do exacrly the same, give the visitor some treats and she soon realises they arent nast people - especially when they have treats

Well he seems to have calmed a little but someone let themselves in again today. He did a low growl, hackles raised slighty, he didn't bark as such it was more of a who are you. I took him to his bed then gave the visitor a treat and Otis came over and took it. Ah well work in progress I just hope there isn't any aggression there

I wouldn't call it aggression, some dogs are more territorial/alert than others. Loki loves people coming into the home whereas outside when it's dark he gets pretty alert and his hackles come up when he sees people from a distance sometimes. Just keep on with the advice you were given and hopefully he'll learn that it's no big deal but if it's just his natural instinct to be wary, it might just be part of his character to an extent. But I wouldn't worry about that because that doesn't make him aggressive.

That's good to know that it isn't exactly aggression. With him being my first dog I am very aware of his behaviour.
In the time from when I last posted we had another visitor which one of us welcomed in and he was great tail going, pushing himself up against the visitor, loving the attention! Makes me think even more he is just being intruder alarm if you like