Rails - Jun 2012 - Creating an object with a nested has_many :through relationship

I have 3 models that define members, groups and the subscriptions: Member, 
Group, and GroupMember.

class Member < ActiveRecord::Base
>   has_many :group_subscriptions, class_name: "GroupMember"
>   has_many :groups, through: :group_subscriptions
>   attr_accessible :email, :password
> end
> class Group < ActiveRecord::Base
>   has_many :member_subscriptions, class_name: "GroupMember"
>   has_many :members, through: :member_subscriptions
>   attr_accessible :name
> end
> class GroupMember < ActiveRecord::Base
>   belongs_to :group
>   belongs_to :member
>   attr_accessible :group_id, :introduction
> end

If I want to create a subscription of an existing member to an existing 
group, I do this in the create method of the GroupMember controller, after 
getting the member and group at first:

@group_member = GroupMember.new(params[:group_member].merge(group_id:
> @group.id))
> @member.group_subscriptions << @group_member

The form just contains an attribute called introduction. This works well.

Now, how can I create a Group with a nested subscription for the current 
user who is filling in the form?

I have tried adding this to the Group model:

accepts_nested_attributes_for :member_subscriptions,
:members
> attr_accessible :name, :member_subscriptions_attributes

In the new method of the Group controller I have this:

  def new
>     @group = Group.new
>     @group.member_subscriptions.build
>   end

So in the new template I added this:

  <%= f.fields_for :member_subscriptions do |ms|
%>
>     <%= render partial: "group_members/form", locals: { f: ms
} %>
>   <% end %>

The form shows correctly with the introduction textarea, and it validations 
are working. But when all data is fine and I submit the form, the new 
template is displayed again with no errors, although I have achieved to get 
the actual error:

[:group_id, "can''t be blank"]


That''s because in my create method I have this (simplified):

@group = Group.new(params[:group])
> @group.save

So the problem is that the subscription is being created from the group, 
when I think it should be created from the member side 
(GroupMember.member_id is not accessible to the member for security 
reasons, as the member could create a subscription for another member, only 
group_id is accessible for him).

How can I create the nested subscription?

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