Documents required regarding accommodation

You do not if you can provide your friend's address and contact information. An invitation is not required but won't hurt either. Both of you will have to go to the nearest police station to register yourselves so be sure to have his ID card/passport handy (and yours too).

There is no need for formal invitation letter, although you will have to declare where you intend to stay during your sojourn in Shanghai. You will give, very precisely the identity of your friend along with the address. After that, you and him/her are under obligation to make a registration at relevant Public security bureau (PSB). Basically, there is nothing complicated in that process. This is quite straightforward and you should not be afraid to do this because it is legal.