All sentences are fine, but you can't say "작가들 중의" or "친구들 중의". You need to say "에" instead of "의" like "작가들 중에" / "친구들 중에". However, if you say just "작가들 중 한명이에요", it's okay. Also, you should care about third sentence because "서울에서" and "서울에서는" have different meaning. If you use "서울에서", that's why you want to say. On the other hand, if you use "서울에서는", it sounds like "the restaurant is popular in Seoul, but the other places are not." Keep it up :)