nested form & habtm

so i am trying to save to a join table in a habtm relationship, but i am
having problems.

= link_to "Create New User", new_user_url(:group => 1)

User model (user.rb)
class User < ActiveRecord::Base
  has_and_belongs_to_many :user_groups
  accepts_nested_attributes_for :user_groups
end

UserGroups model (user_groups.rb)
class UserGroup < ActiveRecord::Base
  has_and_belongs_to_many :users
end

users_controller.rb
def new
  @user = User.new(:user_group_ids => params[:group])
end

in the new user view, i have access to the User.user_groups object,
however when i submit the form, not only does it not save into my join
table (user_groups_users), but the object is no longer there. all the
other objects & attributes of my User object are persistent except for
the user group.

i just started learning rails, so maybe i am missing something
conceptually here, but i have been really struggling with this.