Its not eavesdropping if boss in the IM list. Someone put him there. Either a group member or IT staff if they are actively involved in initial phone setup to include apps. If latter they were told to. I work admin side....we don't do group things just because. Its has a reason given to us, its documented, and done. So that if asked later why is XYZ in the group....we can go because ABC told us to do it.
Was this phone setup by company IT, what apple id(s) are used? We found this gets complicated as phones before issue are setup by us using our generic account. It took a few problems of people sending messages to those not intended to realize our assumption about changing out the apple id would be done by the client was not coming true. So now...we setup with work ID and have clients put theirs in on the spot as they sign for it.
As this can tie in to 3rd part. Some apps will broadcast based on the apple id. 2 users, 2 phones, 2 our generic accounts still in play, 2 people get the message. One of them was not supposed to though. You can play this fun game at home if you own several apple devices and an app that rides on apple id in anyway.
What I'd recommend is get the information of people you actually want to still talk with, save it, wipe the phone and start anew. Odd they let you keep the phone but guess good for a free device. A wipe of the phone gets the company off it though. Depending on how they prepared the phone (apple's tools for example) some attempts at company uniformity/lockdown may be in it. Also avoids ripping apart several apps to see where your problem lays. Or if its in a system setting. Hours potentially to do this...clean slate may work out better really.
Or I'd also recommend you if you can remove yourself from this group or contact someone who can to get it done. You are in a work group it seems but not at that company. Data intended for company use may leak because of this. This is generally a bad thing. Not saying cutoff your old friends from the job. I am saying get a more personalized way of contacting them and off of what seems to be lists with strong ties to the company.