has many relation in a form

Hi,
I have 2 models, group and user.
A group has many users and a user belongs to a group.

In my new group form, I want a checkox list with all my users and get
all the checked users.

How do I do this in the form?

<%= form_for(@group) do |f| %>
...
  <div class="field">
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </div>

    <% @users.each do |user| %>
        <%= f.label user.fullname %>
        ???
    <% end %>
...
<% end %>