Then, have each group present and insure that each group applauds the efforts of the others. I let them sort out who presents first, etc. My goal is to give them as much ownership of the ideas and process as possible. I try to let them know that anything they develop is fair game. Main themes tend to come through clearly and it is not uncommon for certain words to appear in every single one.
Writing styles can vary significantly.
No two may be alike. One can also take this a step farther and share this mission statement draft with employees. A simple, one-page survey works well. Can you operate by this in your daily efforts and use a simple Strongly Agree to Strongly Disagree system. That would seem to be more the exception than the rule.