example.rb

class User
  attr_accessible :full_name
  belongs_to :company
end

class Company
  attr_accessible :name
  
  has_many :users
  has_one :primary_admin, class_name: 'User', conditions: ['users.primary_admin = true']
  delegate :full_name , :to => :primary_admin, :allow_nil => true
end


# admin/user.rb
ActiveAdmin.register User, as: "Users" do
  controller do
    def scoped_collection
      resource_class.includes(:company)
    end
  end

  filter :id
  filter :company_name, as: :string
  
  index do
    column :company
  end
end