Let me get this straight. You want to basically get the lines with the word "user" in them from /etc/groups and /etc/passwd and store them in a file that you can append with your user at a later date?
If so, you need to learn about cat and grep.
If that isn't what you are trying to do, then I am sorry. If that is what you want to do, then it is a simple one line command that you do for what your instructor wants (maybe not simple for you because you haven't done it before). Maybe let us see some of your failed attempts and we can tell you what you did wrong.
Your instructor won't tell you why it is wrong because if he told you, then it would give the answer away because there really isn't much to it.