I'm currently working as a senior'ish engineer, and am being trained to become a junior engineering manager in the next year, so the company's success and mine are intertwined very directly.
About a year back, one of our "ace" engineers, "Jack" (in another division) was screwed over hard (promised a promotion and raise; just got the new responsibilities but no new title or pay; made a point of quitting the day after annual bonus payouts; very messy and political month). Personally, I would never have treated a team member this way, but that's history at this point.
This engineer recently met several employees from our company at a large technical conference. Based on feedback from one of my junior engineers, Jack hosted numerous lunches for his former colleagues, and "connected" them with friends at various companies (i.e. convinced them to work for other companies, but not his own, and basically introduced several of our senior engineers to employers throughout the conference). Over the span of 6 weeks, we've lost 12 senior engineers to various other companies (that is a ton of experience and skills lost). Nearly all of them citing pay bumps, and all of whom joined Jack for lunch(es) during the conference.
Is there any way to address this without having to loop in the legal team? I've identified one of my colleagues as the ringleader (i.e. person who "points people to Jack for a job reference", and am curious why they stay with us while encouraging others to leave), but this doesn't technically violate company policies. I'm debating bringing this up with my own boss, but am worried that if I bring it up now, my boss will be concerned that I took too long to bring it up. I'd like to "stop the bleeding" without getting blamed myself. Leadership already suspects the conference is involved, and regrettably is planning to no longer let us attend it in the future.
Edit: we've asked each departing employee (at least, those that would attend the exit interview; some declined even though attendance is mandatory), "why are you leaving?". It feels like they were all reading off of a script: "need more $$, and I no longer trust this company since I had to threaten to leave to get more money; the counter-offer raise 'poisons the well' and will be my last raise/promotion at this company, so I'm out". Almost word-for-word.