They have the same meaning - they both mean 'except for'. So both sentences mean that everybody came except for Andrea. The difference is that 'bar' can ONLY be used to mean 'except for'. It is not synonymous with 'but' when 'but' is used as a conjunction.

We don't use bar in that way. It's an idiom only used with "none" and it means "without exception." Here are some examples from the link below: She is the brightest student I've ever known, bar none. [=I have never known a student who is brighter than she is] We have the best stadium, bar none, in college football. This is, bar none, the best ice cream I've ever had. My brother-in-law is the single most boring man on the planet, bar none. .................................. http://www.learnersdictionary.com/blog.php?action=ViewBlogArticle&ba_id=92

It's not improper. It's British English.

It is improper English to use "bar" as the way you wrote it. "Barring Adrea, everyone came."

They can mean the same thing, is just another example of two words having the same meaning, depending on how you use them. you could say everyone came (but Angela) ( stayed at home.) everyone came bar Angela.