The Resource Group looks at How to Resolve the Active Directory GUID of a GP Web Client Enabled User Account
The new identity management feature of GP 2013 R2 has quickly become my least favorite feature of R2. It only works with the web client. If you don’t use a SQL user it impersonates DYNSA for the purposes of logging, though at least NOT for security. For example, the user who posted field shows DYNSA regardless of the actual windows user login used. I think this going to be a giant pain for both auditing and troubleshooting.