Is alumni success a valid parameter for selecting a group? I will be starting my Masters degree and intend to be fully committed to working on interesting problems with a lot of impact. I have a few groups in sight which seemingly work on very important problems and publish in major conferences but looking at the 'alumni' section on their website, I haven't noticed MS alumni getting into top PhD programs. Does this say anything about the group's quality or could it be just because of the students?
It is a valid parameter, but not the only valid parameter. It's absolutely justified to ask, and consider, "Where did people in this group go when they were done?"


  I haven't noticed MS alumni getting into top PhD programs. Does this say anything about the group's quality or could it be just because of the students?


You need more information than just "did they get into top PhD programs" - unless a research group is very large, it's likely that any patterns you see in the fates of MS students will be the result of stochasticity. For example:


Are they ending up in private industry? For many people, this isn't a failure, but a success. A group I was in sends many of it's graduate students onward to jobs, and this is considered a success by all involved. Masters programs are not just one step in a PhD conveyor belt. The same is true for "R1 Tenure Track Positions" etc. Many people don't end up taking what is conventionally considered the "brass ring" of their track because they don't actually want to.
Did they want to go to a top tier program and couldn't, or did they not want to? These are two very different things, and will be easier to assess by talking to members of the group, rather than just browsing their alumni page.
Is their alumni page actually accurate? Don't ever assume academic web pages are fully up to date.


It's a valid parameter consider when weighing choices, but I believe it would be a bad idea to eliminate a potential option simply because of their alumni page. You should also look at individuals. Where did the people who presented those important papers end up? Is there someone "like you" who had the trajectory you did? If not, it's worth talking to them about.