If you are asking about Comcast's webmail ("...connect.xfinity.com..."), clicking the "Reply to all" double arrow instead of the "Reply to sender" single arrow should carry along the CC addresses.
Thanks for replying. I am referring to using Xfinity Connect. I couldn't figure it out in my Mac Mail either. I couldn't find the arrows, but when I opened the email and searched for the arrows I found 3 stacked lines alonside of the reply, forward, delete options. Left clicking on it yields a drop down menu w/ a reply all option. That seems to have worked as far as including the chain of cc's in my reply. If I just click on reply the cc box is empty and I assume the email just goes to the sender.