Korean question

1st question : why does everyone put a question mark after 안녕하세여? Isnt it suppose to mean "hello!"
2nd question : to a friend, do we say 안녕하세요 or 안녕?

(answer 1) "안녕하세요?" is not an affirmative statement but an interrogative statement, so we put a question mark. 

(answer 2) We can say 안녕 to our friends.