I've seen this too. It is important to remember that these people are very likely good friends outside of work, talk frequently and openly about work issues, j

I've seen this too. It is important to remember that these people are very likely good friends outside of work, talk frequently and openly about work issues, job prospects, etc., and have been through hell together and back. If things are getting to the point where one person in the group has soured on the company, you can bet they have all been griping about it for months over beers. This in turn sours the rest of the group and then they inevitably start dropping like flies once the first one leaves. In the instance I saw that occur, I think they had their valid reasons, and it wasn't anything horrible about the company they were at--just found something better. But I started placing mental bets with myself on how soon it would be until the rest of the group left--turns out I was more or less right, except for one of them. That said, the impact was definitely felt as they were all super talented people I'd love to work with in the future. So for companies afraid of hiring groups or several close referrals from a single person (which are essentially the same thing), the best thing you can do outside of providing an awesome and competitively compensated place to work is ensure there is solid documentation for things, and when possible ensuring that critical business knowledge won't be lost if someone leaves.