# Models
class Group
  has_many :members, :class_name => "User"
  belongs_to :owner, :class_name => 'User', :foreign_key => 'user_id'
  validate :user_should_only_have_one_group

  def user_should_only_have_one_group
    if owner.has_a_group?
      errors.add_to_base("Bla bla bla")
      return false
    end
  end
end

class User
  belongs_to :group

  def has_a_group?
    !group_id.blank?
  end
end

## Controller
@group = current_user.build_group(params[:group])
@group.owner = current_user

if @group.save
  # ...
else
  # ...
end