RoR - dynamic dropdown with checkboxes

I am looking to implement a dynamic dropdown with checkboxes in it so the sender of a message can choose multiple receivers.

Currently I just have a list of recipients under the message textarea.

_recipients.html.erb:

<% @recipients.each do |user| %>
  <label for="user<%= user.id %>" >
    <span class="list-group-item"><%= user.full_name %></span>
    <%= check_box_tag "message[user_tokens][]", user.id, @message.users.include?(user), id: "user#(user.id)", class: "form-control" %>
  </label>

User.rb:

class User < ActiveRecord::Base

  rolify
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable

  validates :full_name, presence:true,
            length: {minimum: 4 }

  has_many :messages, foreign_key: :sender_id
  has_many :notes
  has_many :homeworks
  has_many :hw_comments
  belongs_to :classmodule
  belongs_to :student

recipient.rb:

class Recipient < ActiveRecord::Base
    belongs_to :message
    belongs_to :user

    validates :user_id, presence: true
end

messages_controller.rb:

class MessagesController < ApplicationController
    before_action :authenticate_user!

    def new
        @message = Message.new
        @recipients = User.all 
        @recipients = @recipients.page(params[:page]).per(8)
    end

    def create
        @message = current_user.messages.build(message_params)

        if @message.save
            flash[:success] = "Message sent"
            redirect_to messages_path
        else
            flash[:alert] = "Something went wrong"
            render :new
        end
    end

    def index
        @messages = Recipient.where(:user_id => current_user.id).order('created_at DESC')
    end

    private
    def message_params
        params.require(:message).permit(:title, :body, :sender_id, user_tokens: [])
    end 
end

Is it possible to get the checkboxs and names in to a dropdown with collection_select for example? Happy to provide more code if required. Thanks.