Procedure to get visa for attending seminar in Hong Kong organized by a third country's company

Dear Sir,
Managing Director and one senior IT official need to attend a seminar in Hong Kong organized by Oracle, USA/India. Do they need original invitaion letter from Oracle or scan copy can be used?

No address is available in the Oracle letter head. Is it OK to submit with the visa application? or e-mail from oracle official can be used?

Please advise us. Rgds. Kashem Khan, Head of IT, Dutch-Bangla Bank Limited, Bangladesh