http://play.golang.org/p/q0VyzeMFOK

main.go

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"strings"
)

type Role int

func (r Role) MarshalJSON() ([]byte, error) {
	if r == 1 {
		return []byte(`"owner"`), nil
	} else if r == 2 {
		return []byte(`"editor"`), nil
	}
	return nil, fmt.Errorf("Invalid role id: %d", r)
}

func (r Role) UnmarshalJSON(b []byte) error {
	roleName := strings.ToLower(string(b))
	switch roleName {
	case `"owner"`:
		r = 1
	case `"editor"`:
		r = 2
	default:
		return fmt.Errorf("Invalid role: %s", roleName)
	}
	return nil
}

type Member struct {
	Name string `json:"name"`
	Role Role   `json:"role"`
}

func main() {
	m := &Member{"Tomoya", 1}
	b, err := json.Marshal(m)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println("Hello, playground: ", string(b[:]))

	b = []byte(`{"name":"Shimaguchi Tomoya","role": "oWner" }`)
	if err = json.Unmarshal(b, &m); err != nil {
		log.Fatal(err)
	}
	fmt.Printf("%+v", m)
}