Poppies are used to remember especially those who have given their lives in battle is because they are the flowers which grew on the battlefields after World War One ended.
If you want to wear a poppy, join. Membership is open, at least for the time being.
Or, maybe you want some extra styles and camoflauge. By joining a clan you will automatically access the clan universal style. Additionally, tour of duty (extra/bonus missions) has been giving away some paint. I can tell you more about free paint. Join.