How do I get a Visa invitation letter?

The Association for Computing Machinery does not issue formal invitation letters for visas. 
They can however  issue a visa support letter. 

For visa support letters, please send all requests to supportletters@acm.org with the following information. 
  • Name (as it appears on your passport) and mailing address 
  • The name of the conference you wish to attend 
  • Include your registration confirmation number 
  • If you are the author of any papers accepted for the conference, please provide the title.
  • If you are a speaker, please provide the title/topic of your speech.
  • Include a fax number or email address of where we can send the letter
Those requesting a letter should allow up to one week to receive it, as requests are handled in the order they are received.

Are reviews single or double blind?

Reviews are single blind. Authors will not know who wrote their review, but reviewers will know whose paper they are reviewing.