Visa Invitation Letter for Tourists.....

Hi ! Im Pavithra and I represent a newly opened travel agency in Sri Lanka. I have a doubt on the visa Invitation or the red seal letter. We have 2 passengers who need to visit China in Late November.

Apart from the general documents what is the invitation letter required? Is a general invitation letter sent by our partner travel agency in China is enough or do we have to get it from Tourist Bureau or a Authorized institution in China?

Some say we do not need such letter if there is group of 5 travelers... Is this true.... Please help. Thank You.