Early registration is highly encouraged to allow enough time for your visa application to be processed.
International attendees are responsible for acquiring their own visa, as needed. Complete information and a link to apply for a Visitor Visa Business B-1 for conference attendees can be found here: https://travel.state.gov/content/visas/en/visit/visitor.html
If you have questions about the visa application process, please refer to your local U.S. Embassy or Consulate: https://www.usembassy.gov
An invitation letter is NOT required to apply for a U.S. Visa. Per the U.S. State Department website:
Visa applicants must qualify on the basis of the applicant's residence and ties abroad, rather than assurances from U.S. family and friends. A letter of invitation or Affidavit of Support is not needed to apply for a nonimmigrant tourist visa. If you do choose to bring a letter of invitation or Affidavit of Support to your interview, please remember that it is not one of the factors that we use in determining whether to issue or deny a nonimmigrant tourist visa.