Se retrouver is more like "meet again". Se rencontrer is to meet. I can also use Se retrouver to say "we are planning to meet in a particular place. You can also use Se rencontrer for love affairs. You can also use those expression not involving people, more often Se retrouver than se rencontrer. Le sujet que j'ai abordé se (re)trouve dans le livre "..." Ce genre de cas précis se rencontre dans telles circonstances...