They are same. 친구와 함께 영화관에 갔다. 친구와 같이 영화관에 갔다. I went to the movie theater with my friend. As for 'formal', it doesn't matter which one you use it.

함께 and 같이 are almost same in the meaning of "together(with)" Both are formal. But 같이 has another meaning 'like' Refer to http://www.italki.com/question/92819