哥们 is very informal, used mainly among male friends. It's a northern/Beijing expression. The variant 哥们儿 (gēmenr) with erhua is also common.
usage
Can be used as a form of address (哥们！= Bro!) or to refer to a close male friend. Women typically don't use this for each other — the female equivalent is 姐们 (jiěmen).
