To ask three or more people how they are we use the same sentence structure we do for asking one or two people, but we replace 'koe' or 'korua', with 'koutou' so:
Kei te pehea koe? (How are you?) to one person, and Kei te pehea korua? (How are you?) to two people becomes:
Kei te pehea koutou? How are you? to three or more people.