ACM does not issue official invitation letters, but they do issue visa support letters. Below is visa information that might be helpful for international attendees:
The sites http://www.unitedstatesvisas.gov & http://travel.state.gov have information about obtaining a visa for those traveling to the United States. Both sites have links to websites for U.S. embassies and consulates worldwide. The embassy and consulate websites have very helpful information about procedures, timelines, communities served, required documentation, and fees.
International registrants should be particularly aware and careful about visa requirements, and should plan travel well in advance. All visa inquiries must be handled by ACM HQ. Please send your request for a letter in support of a visa application to supportletters@acm.org. Include your name, physical mailing address, fax number, and the name of the conference you are attending. Authors of papers/posters should include the title. Visa support letters cannot be issued via email.
Please note that ACM does not issue formal "letters of invitation" to any of its conferences.