"Why don't you go out with us tomorrow night?"

"Why don't you go out with us tomorrow night?"
 

One way of inviting someone out is to say "Why don't you go out with us tomorrow night?"

Would a direct translation into Spanish give the same feeling:

"Porque no salgas con nosotros manana por la noche?"

"Salgas" is a subjunctive, but there is no need for that. Indicative present is fine. :)

Is it incorrect to use salgas? or just preferable not to? In other words, do people sometimes use it as I've written above?

It's incorrect. It is not a situation that calls for the subjunctive. Sorry for not making it clear. ;(