"¿Ustedes van a salir sobre las doce?"
Translation:Are you going to leave around twelve?
'Are you leaving' would present tense while this question expresses the future so its important to in include the translation of 'van a' (going to) in your answer.
To ask more then one person, 'are you leaving' I believe is ustedes salen?
In speaking of hours; "At what time..." , "At twelve o'clock" > AT AROUND TWELVE Where have those disappeared?
This is a good connotative understanding, ekihoo, but I don't know what you mean by the word "disappeared."