With LC ES update 1, you'll be able to use a user service to get that information.
In the current build, you can do an API call or use the JDBC service to query the database.
You can do something like SELECT * FROM edcprincipaluserentity where refprincipalid = 'GUID'
Jasmin, which service do I call to retrieve the real user ID, given the GUID/ I have update 1 now, but don't see which service to use.
nevermind, I found out. Using the UserManagerLookupService and the Fidn User service operation, I passed in the Task - User's ID which is the GUID. I created a user variable in the custom render process and assigned the output of Find User to that variable. I then used a SetValue service to set the user's user ID (real login id) to another variable of type string. It works great.