Rails 3 has_many :through form with dropdown menus?
class Project < ActiveRecord::Base
has_many :employees, :through => :teams
has_many :teams

class Employee < ActiveRecord::Base
has_many :projects, :through => :teams
has_many :teams

class Team < ActiveRecord::Base
belongs_to :project
belongs_to :employee

<%= f.input :employees, :as => :select %>

<%= f.semantic_fields_for :employees do |builder| %>
     <%= render 'employee_fields', :f => builder %>
<% end %>
<%= link_to_add_fields "add employee", f, :employees %>

<p class="fields">
    <%= f.input :name, :as => :select, :collection => Employee.find(:all) %>
    <%= f.hidden_field :_destroy %>
    <%= link_to_remove_fields "remove", f %>
</p>

<%= semantic_form_for @project do |f| %>
    .
    .
    <%= f.semantic_fields_for :teams do |builder| %>
        <input id="project_teams_none" name="team[employee_ids][]" type="hidden" value="" />
        <%= render 'team_fields', :f => builder %>
    <% end %>

<div class="input">
    <%= f.collection_select(:employee_id, Employee.all, :id, :name, :include_blank => true ) %>
</div>