Inviting coworkers to an upsherin is something I find a bit odd- do you also invite your coworkers to your son’s 4th birthday party? Or to your 10 year old son’s haircut? Should I have invited my associates to my last haircut (what little I have left)? Hey, Rob, Dave, and Jim- let’s go to the barber shop for my haircut, then we’ll go to the Rabbi’s house for some honey covered books. Or is it only if I get a haircut on my birthday?
An upsherin is very different from a bris, bar mitzvah, or wedding. And in my experience, inviting your close coworkers or business associates to a bris, bar mitzvah, or wedding that you are making is almost expected (from their point of view- so darchei shalom), and is always an opportunity to make a kiddush hashem. You don’t have to, but I think it’s pretty understandable why people who do invite coworkers are doing so.