Informal - if you are addressing a peer or a younger person:
"Kumusta ka?" (Singular)
"Kumusta kayo?" (Plural)
Formal – if you are addressing an older person, a superior, an official, or a stranger, who appears to be not much younger than you:
“Kumusta po kayo?” or “Kumusta po sila?” (Singular and plural)
At times “kumusta” is written as “kamusta”.