I want to know what document do I need to prepare for my family visiting me on my graduation.

Dear Sir,
I am an international student in Communication University of China. My graduation date is 28 June, 2018.
I want my family to visit me on my big day. What documents should I submit for their visit to me. They will stay here for a week, probably. Unlike my mother and siblings, I came to China directly from Canada with my Bangladeshi passport for which I need not show any flight itinerary. And they will live with me in here too. So, I was wondering if I write a letter on my own, would that suffice or do I need to ask my professors to write one. Moreover, do they need to purchase plane-tickets before they even apply for the visa? What other papers might they need?
Thanks in advance.