너네 = 너 (you) + -네 (suffix for "family" or "group" the person belongs to). * 너네 is a casual term only appropriated for use with close friends or toward children. * 너희 is a little more formal and literary, with the same politeness level as 너네. Both are standard phrases (https://twitter.com/urimal365/status/542908733064376321). (But 너네 is not listed in the dictionary because it's not a word but a compound form of 너 + suffix 네) - 너네는 식구가 몇이야? (How big is your family?) == 너네 집[가족]은 식구가 몇이야? - 걔는 멋져 (He's cool) => 걔네는 멋져 (Those guys[they] are cool) == 걔네들은 멋져.

"너네" is not standard Korean. but Korean often use it. Standard Korean of "너네" is "너희". it means you. and "너희" is the plural form of "너". Generally elder says "너희" to younger persons. It is very impolite that young person says "너희" to older persons.