UndeIn Designer, under the Custom palette there is a Process Fields object. This object contains a number of fields that the process engine uses to keep track of the state of the process. Drop this object onto your form (there are a number of hidden fields). The AWS_ASSIGNED_ID has the GUID of the user that the task is assigned to. You can take that value and use the as input into the User Lookup service under the Foundation category to get the uid or username or whatever you want.