First, I imagine you will not be required to write this summary until you actually have the "group discussion" (either in class or outside of class). To prepare for your reflective summary, I suggest you take notes as your group talks. It will help to have a list of the questions or issues that you are supposed to discuss, which I imagine your teacher will provide in advance. Under each question, write 2-3 conclusive points your group discusses. You can also jot down any questions that come up which remain unanswered.
Then, when it is time write the reflexive summary, the first thing you will do is review your notes. A "summary" is typically less formal in structure than a full "essay" so it is acceptable to write write a short paragraph on each point discussed. If you feel many answers are repetitive (even though they are about different topics in the discussion) it is perfectly logical to group similar answers together.
Reflective summary suggests that you also add some personal reflection into what was discussed. This means that your summary can include your own opinions. Include ideas that you highly agreed with, and why. Also, feel free to include ideas that you disagreed with, and why. To conclude the entire summary, you might want to wrap up with an overall idea of the most important thing you learned or took away from the discussion.