A visa is normally required to visit China. You can find out if you need a visa by going to the embassy/consulate of P. R. China in your country and apply for it if you do.
If you need an invitation letter for the visa, after register at JACoW/SPMS, under Delegate Registration Receipt, you will find a link back to the conference website. You can dowload the invitation letter after input your personal information (full name, date of birth, passport number and etc.).
If you still have questions on how to get the invitation letter, please contact: email@example.com.