What idea does Meg share with Calvin to try to free Charles Wallace from the mind control of IT?

Meg tells Calvin to use his gift of communication to try to free Charles Wallace from the control of IT. You can read all about this in chapter 8.
In that chapter, both Meg and Calvin realize that Charles Wallace is a shadow of himself. Frustrated at their inability to help him, they each take hold of Charles. However, Charles is now far stronger than either Meg or Calvin. He manages to break free. Both Meg and Calvin keep trying but are eventually overpowered by the bodyguards of the Man with Red Eyes.
For his part, Charles Wallace makes a terrible comment that frightens Meg and Calvin. He suggests that, perhaps, Mrs. Whatsit, Mrs. Who, and Mrs. Which are the true enemies. Charles Wallace even goes so far as to advise Meg and Calvin to submit peacefully to the powers in charge.
Eventually, Meg suggests to Calvin that he should use his powers of communication to bring Charles Wallace back. In chapter 5, we learn that Calvin has a great gift. In that chapter, Mrs. Whatsit tells Calvin that she would strengthen that gift. So, in Chapter 8, this is the gift Meg is referring to when she suggests that Calvin use it to save Charles Wallace.