habtm user interface; unexpected behaviour (calling update instead of method?)

I'm trying to get a user-interface going, where the user can choose
which users he/she would like to add to a group. edit.html.erb for the
Group displays a list of users currently assigned to the group we're
editing, as well as a list of users not assigned to this group yet. On
both lists there are buttons to assign a user to the group resp. to
remove the user from the group. (see code below).

This works. Almost. There are some unexpected behaviours that puzzle
me:

- the topmost row on both lists cannot be moved to the other list.
When clicking the '>' or '<' button next to the topmost-listelement
the application executes an "update" command instead of adding or
removing the user via the action => command that is assigned to the
button. (when I sort the users according to id intead of names, it is
still the top-row that can't be moved with the buttons; it must have
something to do with the position, I think)

- the update-button as well as the "show" and "back" links are being
displayed at the very top, even though, in the code they follow after
the lists... an indication that I'm doing something fundementally
wrong, it seems to me.

can you get me back on track? Any comments and improvements are very
welcome!
thank you for your time
haddock

ps I'm planning to use partials later on, but for the moment I would
like to get this working first

---------------------------------------------------------views/groups/
edit.html.erb
<h1>Editing group</h1>

<%= error_messages_for :group %>

<% form_for(@group) do |f| %>
  <p>
    <b>Name</b><br />
    <%= f.text_field :name %>
  </p>

   <p>
  <table>
  <tr>
    <th>Users of this group</th>
    <th></th>
  </tr>

  <% for user in @users_of_group %>
    <tr>
      <td><%= button_to '>', :action
=> :remove_user_from_group, :user_id => user.id, :group_id =>
@group.id %></td>
      <td><%=h user.id %></td>
      <td><%=h user.name %></td>
    </tr>
  <% end %>

<table>
  <tr>
    <th>Users available</th>
    <th></th>
  </tr>

  <% for user in @users_not_of_group %>
    <tr>
      <td><%= button_to '<', :action => :add_user_to_group, :user_id
=> user.id, :group_id => @group.id %></td>
      <td><%=h user.id %></td>
      <td><%=h user.name %></td>
    </tr>
  <% end %>
    </p>

  <p>
    <%= f.submit "Update" %>
  </p>
<% end %>

<%= link_to 'Show', @group %> |
<%= link_to 'Back', groups_path %>

---------------------------------------------------------excerpt from
groups_controller.rb
  # GET /groups/1/edit
  def edit
    @group = Group.find(params[:id])
    @users_of_group = @group.users.find(:all, :order => "name")
    @users_not_of_group = User.find_by_sql(["select * from users where
id not in (select user_id from groups_users where group_id = ?) order
by name", params[:id]])
  end

  def add_user_to_group
    @user = User.find(params[:user_id])
    @group = Group.find(params[:group_id])
    @group.users << @user
    redirect_to :action => :index
  end

  def remove_user_from_group
    @user = User.find(params[:user_id])
    @group = Group.find(params[:group_id])
    @group.users.delete(@user)
    redirect_to :action => :index
  end

I had a similar problem where things were working fine in everything
but IE. It was solved by adding <tbody> tags into my tables. Not
sure if that's your problem but I thought I'd mention it.

Thanks for your comment. I did as you told plus I've cleaned the html
a bit, there were some errors as well. The second point about the
placement of the button/links is now working as expected!

This still leaves the first issue.The fist element/user still wont
remove itself from the list, if I press the arrow-button next to it.
Instead the "update"method gets called. With other users/list-items it
works just fine. Below the html code for the table, as well as the
update-method. The update-method is a by-product from the scaffold
command. Maybe to someone who understands it better than I it is
obvious, why it is called when the top-button of the following table
is pressed?

***---------- update method in controller
  def update
    @group = Group.find(params[:id])

    respond_to do |format|
      if @group.update_attributes(params[:group])
        flash[:notice] = 'Group was successfully updated.'
        format.html { redirect_to(@group) }
        format.xml { head :ok }
      else
        format.html { render :action => "edit" }
        format.xml { render :xml => @group.errors, :status
=> :unprocessable_entity }
      end
    end
  end

***----------html-source of table
<table>
<thead>

  <tr>
    <th>Users of this group</th>
    <th></th>
  </tr>
</thead>
<tbody>

    <tr>
      <td><form method="post" action="/groups/remove_user_from_group?
group_id=3&amp;user_id=3" class="button-to"><div><input type="submit"
value="&gt;" /><input name="authenticity_token" type="hidden"
value="339fe1a43ed7259223d4812901af2400909ab073" /></div></form></td>
      <td>3</td>

      <td>abcdefghij</td>
    </tr>

    <tr>
      <td><form method="post" action="/groups/remove_user_from_group?
group_id=3&amp;user_id=2" class="button-to"><div><input type="submit"
value="&gt;" /><input name="authenticity_token" type="hidden"
value="339fe1a43ed7259223d4812901af2400909ab073" /></div></form></td>
      <td>2</td>
      <td>aeruoweuroweur</td>
    </tr>

    <tr>
      <td><form method="post" action="/groups/remove_user_from_group?
group_id=3&amp;user_id=1" class="button-to"><div><input type="submit"
value="&gt;" /><input name="authenticity_token" type="hidden"
value="339fe1a43ed7259223d4812901af2400909ab073" /></div></form></td>
      <td>1</td>
      <td>chaljdfaf</td>
    </tr>

</tbody>
</table>

solved!

I shouldn't have put the <%= button_to '> elements within another
form. Nested forms are no good, it says so on page 476 in the bible
(agile web development with rails).

cheers