Habtm + accepts_nested_attributes_for

Rails
1

so i have read conflicting information about this, some seem to pull
it off, some say its impossible, but here is what i am trying to
accomplish.
i am using authlogic w/ lockdown, and when i create a new user, i want
to add an entry into the user_groups_users join table with the new
user id, and a group id i pass it. i have tried several variations,
but here is what i ended up with (edited down to only pertinent info).

User model

class User < ActiveRecord::Base
  has_and_belongs_to_many :user_groups
  accepts_nested_attributes_for :user_groups
end

UserGroup model

class UserGroup < ActiveRecord::Base
  has_and_belongs_to_many :users
end

users_controller

def new
  @user = User.new(:user_group_ids => params[:group])      # gets
passed in from /users/new?group=1
  @user.user_groups.build    #i have tried with and without this.
with it, i get duplicate ID fields in my view
  end

view form (haml)

- form_for @user do |f|
    - f.fields_for :user_groups do |g|
      = g.text_field :id

if i fill out all the required fields, the user fields are saved, but
no user_groups_users entry is made
if i sumbit the form with just the user_groups_user id, the field
dissapears, but in my params, i see

    user: !map:HashWithIndifferentAccess
      user_groups_attributes: !map:HashWithIndifferentAccess
        "0": !map:HashWithIndifferentAccess
          id: "1"

i admit, i am a r00bie, so maybe i am going about this entirely
wrong. but can anyone offer any input?

thanks