Tense of the verb
That doesn’t seem right to me. If it were ‘The boss and his colleagues’ then yes, ‘have’ is correct. But the subject of your sentence is just the boss, so ‘has’ is correct. Think of it this way: you could rearrange the sentence into ‘The boss has reached the hall, with his colleagues.’
So ‘The boss, with his colleagues, has reached the hall’, is surely correct.
Edit: accidentally put ‘object’ in place of ‘subject’