For he/she is happy you would say "(El/Ella) està contento(a), depending if it's a male or a female the person you're talking about. I put El/Ella in parenthesis because you don't always use the definite article to make phrases, specially if you just mentioned the subject before, for example: "Hoy hablé con mi amiga Ana, està muy contenta".
For plural: "(Ellos/Ellas) estan contentos(as)".