お帰りなさい probably comes from something like "よくご無事にお帰りなさいました" in old days (like Edo era). So it is expressing your relief that someone came back safely again. You can use it whether if it's home, or region, or any group/entity. If your classmate came back to the class from a year study abroad, you can also welcome her by saying "お帰りなさい”.

If you are welcoming someone at some place where is not their home , I think it would not be "welcome someone back" but "welcome someone" , right ? It will depend on specific conditions , usually we would say "ようこそ＋some place＋へ" . For eg. 「ようこそsony東京本社へ」or 「sony東京本社へ、ようこそ」 . At the restaurant , you can always hear the waiter say : 「いらっしゃいませ」. This is a kind of "honorifics" in Japanese .

Of course you can , especially between friends or relatives . If the listener is your boss or someone important , you need to say "お疲れ様です/でした" .