* "I am friends with X." This is the most common and natural way to express this idea.
* "X and I are friends." This emphasizes the mutual friendship.
* "We are friends." This is less specific but still correct, assuming the listener understands who "we" refers to.
"I am friend with X" is grammatically incorrect because it uses the singular form of "friend" when referring to multiple people.
"I am a friend of X" is grammatically correct but less common than the first option.