Has many relation in a form

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| %>

<%= f.label :name %>
<%= f.text_field :name %>
<% @users.each do |user| %>
    <%= f.label user.fullname %>
<% end %>

<% end %>