gruntfile-amd-concat.js

concat: {
  amd: {
    src: "tmp/**/*.amd.js",
    dest: "dist/my_library.amd.js"
  }
}

gruntfile-amd.js

transpile: {
  amd: {
    type: 'amd',
    files: [{
      expand: true,
      cwd: 'lib/',
      src: ['**/*.js'],
      dest: 'tmp/',
      ext: '.amd.js'
    }]
  }
}

gruntfile-browser.js

grunt.registerMultiTask('browser', "Export a module to the window", function() {
  var opts = this.options();
  this.files.forEach(function(f) {
    var output = ["(function(globals) {"];

    output.push.apply(output, f.src.map(grunt.file.read));

    output.push(grunt.template.process(
      'window.<%= namespace %> = requireModule("<%= barename %>");', { 
      data: {
        namespace: opts.namespace,
        barename: opts.barename
      }
    }));
    output.push('})(window);');

    grunt.file.write(f.dest, grunt.template.process(output.join("\n")));
  });
});

// ...

grunt.initConfig({
  // ...
  browser: {
    dist: {
      src: ["vendor/loader.js", "dist/my_library.amd.js"],
      dest: "dist/my_library.js",
      options: {
        barename: "my_library",
        namespace: "MyLibrary"
      }
    }
  }
})

gruntfile-commonjs.js

transpile: {
  // ...
  commonjs: {
    type: 'cjs',
    files: [{
      expand: true,
      cwd: 'lib/',
      src: ['my_library/*.js'],
      dest: 'dist/commonjs/',
      ext: '.js'
    },
    {
      src: ['lib/my_library.js'],
      dest: 'dist/commonjs/main.js'
    }]
  }
}

gruntfile.js

module.exports = function(grunt) {
  grunt.loadNpmTasks("grunt-es6-module-transpiler");
  grunt.loadNpmTasks("grunt-contrib-concat");

  grunt.initConfig({
    transpile: {
      amd: {
        type: 'amd',
        files: [{
          expand: true,
          cwd: 'lib/',
          src: ['**/*.js'],
          dest: 'tmp/',
          ext: '.amd.js'
        }]
      },

      commonjs: {
        type: 'cjs',
        files: [{
          expand: true,
          cwd: 'lib/',
          src: ['my_library/*.js'],
          dest: 'dist/commonjs/',
          ext: '.js'
        },
        {
          src: ['lib/my_library.js'],
          dest: 'dist/commonjs/main.js'
        }]
      }
    },
    concat: {
      amd: {
        src: "tmp/**/*.amd.js",
        dest: "dist/my_library.amd.js"
      },
      browser: {
        src: ["vendor/loader.js", "dist/my_library.amd.js"],
        dest: "dist/my_library.js"
      }
    }
  });

  grunt.registerTask("default", ["transpile", "concat"]);
}

my_library.js

import { shout } from "./my_library/shout";
import { ssshh } from "./my_library/ssshh";

export { shout, ssshh };

package.json

{
  "name": "my-library",
  "main": "dist/commonjs/main.js"
}

shout.js

var shout = function(s) {
  return s.toUpperCase();
}

export shout;

ssshh.js

var ssshh = function(s) {
  return s.toLowerCase();
}

export ssshh;

A word of caution about using grunt.template.process here. It will mangle your underscore template strings if you have any in your module code.

We've switched it to

grunt.file.write(f.dest, output.join("\n"));

running grunt transpile:commonjs has the error as follow:

Running "transpile:commonjs" (transpile) task
>> lib/my_library/shout.js: [L5:C8] Unexpected identifier
Warning: Error compiling lib/my_library/shout.js Use --force to continue.

Aborted due to warnings.

line 5 should be revised as export { shout }, and then grunt running successed.