"Besides" suggests that enhancing (not inhancing) your English isn't a central purpose to volunteering to make new friends. It is a secondary purpose. But if learning English is as important as making new friends, then "in addition" would work better. If the two elements are about equal in importance, use "in addition." I missed the movie because I arrived late. But I wasn't too upset because friends later told me it wasn't very good. Besides, I was hungry so I went and had dinner instead.