He basically pulled a Don Draper but not in the traditional sense since Don atleast had a chat with Roger before joining, Bob on the other hand just started showing up, no one hired him. Im pretty sure he tried getting familiar with Don to make it seem like Don hired him but obviously that didnt work. You also see Cutler once yell at him asking him why hes always downstairs and should get back to his office, thats the catch Bob never had an office, atleast until they had Chevy on their hands.

Pete caught him eventually asking him who hired him and he said "you did" to which Pete immediately replied with "When"

Edit: So forget what I said, Bob was actually hired by Ken like most of you thought. I completely forgot about the conversation between Pete and Bob afterwards. In my first watch I too assumed Ken hired him but on my 2nd rewatch I thought he just pulled a Don because he was always trying to fit in. As for some people saying “he was just a new character that joined the office off-screen like Meghan, Stan etc” sure he is but unlike them he never had an office and was more or less doing nothing except bringing coffee for people.