If referring to a group of people I would just say HI or HELLO. Down south in the United States people say "Hi ya'all" meaning "hi you all"

As a greeting, either would work. But I might punctuate the second thus: "Hi, all!" As Valerie notes, the "all" is generally unnecessary, and I'd only use it if in the context the greeting could otherwise be taken as addressing just one person when several people were intended.