You can find out if you need a visa to visit the United States, as well as details on the expected time and process, using U.S Department State - Bureau of Consular Affairs website.
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If you require a letter of invitation to support a visa application, please be sure to provide sufficient lead time for the processing of the letter. The organizers request that these participants register early. A letter of invitation will be sent from the conference organizers upon providing a receipt of the paid conference registration fee and placement on the program.