I see you already have the meaning of "à" in Bonjour à tous (+ et à toutes)". In Italian we say "Buongiorno a tutti (+ e a tutte)", in English you could also say "Hello to everyone". So when you hear "Hi everyone" it's not a different grammar: it's a phenomenon called "ellipsis" which means "hiding". The "to" still exists, but you can't see it. Why? Generally, to say the thing faster. Why Arabic does not write all the vowels? Because it's not strictly necessary in some context. Hello TO everyone is just more "grammatically pedant" . PS -- other meanings of "à"? yes there are: for describing some Objects, it means "for the purpose to" like "Machine à laver" (washing machine) et "machine à glaçons" (ice machine). PS 2 -- From another point of view, they could be different sentences with a different grammar. Thinking about "Salve Regina!" we notice that Regina, here, is not an Indirect object (with an ellipsis of the TO) but a Vocative-complement, generally preceeded by "O" (Oh Regina), or with the exclamation tag, or both (Oh, Regina!) Then we could write ---Salve alla Regina// or also ---Salve a te, Regina!// where we have used both the complements. So if I say ---Hello, everyone!/ I have used the Vocative; and in ---Hello to everyone/ the Indirect object; and lastly in ---Hello to you, everybody! // I have used both. In French that could be: ---Bonjour, mr.Andrea!/vocative; ---Bonjour à mr.Andrea/indirect obj.(it sounds as a third person, so that should be extremely formal, like facing an emperor); and ---Bonjour à vous, mr.Andrea. which perhaps is the best one.