你们好朋友! 你们好吗! - × should be 朋友们! 你们好吗 !
你好! = "Hello" (only to one person) - √
你们好! = "Hello" (when saying it to a group) - √
你好,朋友! = "Hello friend" (but this CANNOT mean "hello friends"). - √ or 朋友你好!
你们好,朋友! = "Hello friends" - a little strange, better say '朋友们,你们好', or '你们好,朋友们', or just '你们好'
你好吗? = "How are you?" (only to one person) - √
你们好吗?= "How are you?" (to multiple people) - √