Mind your own business - it's not your problem.
If someone working in your group suggests or does plagiarism, or someone tries to plagiarize your work, or does something which could affect your mark or your academic record, go ahead and report them.
But you shouldn't get involved with this. It's not your job to police other students behaviour. It doesn't prevent you from learning, nor does it prevent you from getting the grade you deserve.
You are laying yourself open to unpleasantness and ill-feeling between you and your classmates. Other students who are honest, might resent you or feel that you will be surveilling them in future. You might end up with your own behavior being closely watched by classmates who are trying to get you in trouble. It is also possible that the information you have is wrong or that you have misinterpreted it. For someone who is innocent of cheating or who has been co-erced to be disciplined for plagiarism is a serious thing to have on your conscience. In some (rare) situations various forms of cheating are tacitly being tolerated by the teacher, and you will be seen by them as 'rocking the boat'.
In short, you have a lot to lose and very little to gain by reporting them.