one.go

package main

import "fmt"

func main() {
	user := User{}

	// Set the group:
	groupSetter("admin", user)

	fmt.Println(user)
}

type User struct {
	Name  string
	group string
}

// GetGroup returns the group the user belongs to.
func (user *User) GetGroup() string {
	return user.group
}

func groupSetter(name string, user UserInterface) error {
	return user.SetGroup(name)
}

//
type UserInterface interface {
	GetGroup() string
	SetGroup(name string) error
}