In a 1x1 with my manager the other day we discussed an interesting topic, which I don't think either of us had fully recognized in this way before.
It seems somewhat obvious that adding a really talented person to a team can bring up the total productivity to a higher level than you expected. For purposes of thought experiment, say four folks are on a team and you add a really strong additional person -- you might get 5.5 or 6 people of expected effort and finish tasks more quickly, or complete the same tasks at much higher quality than expected.
What's not always so obvious is that the opposite is true -- if you have 4 folks on a team and you add a weak additional team member, you may not just lose the work of that one person (unless everyone else isolates them entirely). You could lose some effort/quality from the others as they help out, either by assisting the weak person or by outright doing their work to compensate. So instead of what you'd expect from a team of five people, or even four people, you might only get the results of a team of 3.5 or 3 people.
This could be a short-term concern, everyone has some weaknesses when they first join a team even if it's just lack of familiarity with the other team members or specific work, but something to keep an eye on over time.
Great managers understand how to put together teams and match them against work to gain these multipliers from the start. I'm going to focus on recognizing when these multiplier effects are happening and reinforce the positive gains, and make changes when teams are having negative mutiplier outcomes.