Before traveling to the U.S., citizens of many foreign countries need to obtain a visa. Detailed information is available in the U.S. Department of State, Visa Section:
It is the sole responsibility of the individuals, who would like to attend the conference, to apply and receive proper visa to enter the United States. If you need an entry visa, you must apply in advance and allow adequate time for your visa application to be processed. We strongly recommend you to apply for visa 3-4 months before the conference.
An invitation letter will be sent to you only after the registration fee is paid and your registration is complete. Select visa letter required when filling out your registration via the conference registration portal. You will be sent the visa letter within 1-2 business days of paying your full registration fee. If you have questions, or have not received your letter, please email Rebecca Krishnamurthy at firstname.lastname@example.org.