Working together is fine. Discussing with your classmates things like what order of topics is best and what appendices to include is fine. Each person should write his or her own report. There are many ways to do this. Look in the book at examples if you aren't sure how to phrase something.
Active voice is easier to read.
Ask a person to read the report out loud to you. You will hear awkward phrasing, run-on sentences and misplaced punctuation. Lastly, refer the reader to the attached report for more information. Don't forget to date and sign your letter. Think about the order of topics in your report.