Who is the Site Guest User?
The Site Guest User is a special type of user that you use in conjunction with a Force.com site or any application that utilizes a Force.com site. Some examples of this are event management applications, donation applications, volunteer applications, etc. The Site Guest User allows for those external constituents who are registering for an event, making a donation, signing up to volunteer to commit data into your system, essentially making the “sync” of data possible.