"just" is being used for emphasis here and means something like "exactly". If you left it out, it wouldn't really change the meaning of the sentence very much but what the writer is trying to convey is that it has always been known that friendships are important for older people but the new studies are making this clear in a more objective way, presumably by quantifying the effect of friendships on longevity or quality of life in some way. "more so" means the same as "more" here (to me, it would actually sound more natural without the "so") - there is an article on "more so/moreso" at http://grammarist.com/usage/moreso/