East Bay slang term, indicating familiarity with and affection towards any given subject. Can also be used in the negative, as in "I'm SO not friends with that" to express distaste for the subject. Similar to being down with that.
"Stay home and smoke a blunt with you instead of going to school? I'm hella good friends with that."
"Dude, they wanted me to stay until midnight, sitting on my thumbs. I'm NOT friends with that."