Plagiarism is presented to students as a binary concept: either you're doing it or you're not, and you'd better not, because if you do it there are clearly defined repercussions meted out by a clearly defined group of people, the academic honesty personnel at the university.
In post-student academia, the truth is that plagiarism functions more as a continuum. Instead of one set of rules that all the world's academic members agree upon, there are less precise (though no less deeply held) cultural norms that most of the world's academic members mostly agree upon. There is also no good analogue in the "real" academic world of "academic honesty personnel": instead, academia is largely self-policing.
So in your case, instead of asking "Is it plagiarism?" you should be asking "Is this an academic best practice? If not, how bad is it? Is it actionably bad?"
In the case at hand:
[T]his new article has many (more than 8) sentences taken from the older article from the same research group. These sentences describe the experimental setup. It's clearly the same setup as the older article.
My view on this is: it's far from being bad enough to merit any outside corrective action. Because the word "plagiarism" sounds very serious and we in academia have a stake in keeping it that way, I would not use that word in this instance. (And if you do, your colleagues / superiors / affiliates may think you're overreacting.)
Three key points:
Plagiarism is defined as using the ideas and/or distinctive language of someone else. I gather from your description that the two papers were written by the same group. (You say "with a different first author". I don't see the relevance of that -- all the authors of the paper are all the authors of the paper when it comes to issues of academic integrity and citation.) Recycling your own work is a different academic crime, often called "self-plagiarism," but I discourage people from using the term "plagiarism" to describe it: it gives the wrong idea.
You say that they cite their older paper. That's a key point in their favor -- it means they are not trying to fraudulently pass off old work as being new. One can even argue that the paper went through the refereeing process and the referees and the editors apparently had no issue with the repetition. (That doesn't necessarily make it okay, but it provides some useful perspective.)
The essential currency of academic work is intellectual novelty, but that does not mean that everything that appears in an academic work is or should be intellectually novel. There is a certain amount of routine, procedural stuff that needs to be there, but that most expert readers will quickly pass their eyes over.
I hope you notice that at no point did I claim that the authors have followed best practices here. I do think it's lazy to lift multiple sentences and whole paragraphs from a previous work. In my opinion, even if something is completely routine and what you write is going to read the same way as what many other people have read before: okay, then you can type up a new, uncopied version quickly and easily. Also, as a reader, when you catch someone copying anything, your opinion of them and the novelty of their work goes down a bit.
I will end with this: I remember once, some years ago, reading a paper of a certain student of one Professor A and having the feeling that the paper was morally similar to a paper of Professor A's that I had glanced at before. When I compared the two papers side by side, I found that not only was the intellectual content closely analogous, but the student had evidently written the introduction to his paper by starting with the introduction to Professor A's paper and making close to the minimum possible amount of change necessary for the same text to introduce his own paper. I did not seriously consider pursuing a plagiarism charge on the student...but it left me with a negative impression of his creativity, independence and work ethic. From what you say about the two papers, the main issue is not that some routine procedural passages were copied, but really that the second paper is rather derivative on the first -- not fraudulently so, but in a way that makes the second paper not that interesting to you. That you think this about the authors' work is, I think, the most appropriate negative consequence of their actions.