You might need a visa to travel to US, see: http://icra2015.org/travel-registration/visa-information.
In order to get an invitation letter for the visa application, please provide the following information of the participant:
- full name and contact (email and address)
- copy of registration receipt for ICRA 2015
- copy of passport information page
- name of the workshop
This information can be emailed to email@example.com and the conference organizers will generate a letter and send it back by email. Please write ICRA_CPG_WORKSHOP_VISA in the email subject field.