Sharepoint task list - 'Assigned To' field returns userid instead of username/email address

I am working on trail version of nintex workflow for sharepoint online site and I have question regarding one of the scenario

-         I am trying to get the “Assigned To” field value from one of the task list item

-         In the workflow I get user id (integer value) as returned value instead of username or email address.

-         How can I go about to retrieve associated username/email address based on the id returned?

-         I tried to use <site url>/_vti_bin/usergroup.asmx web service and dictionary object but everytime I got 0 count returned in the response. I was getting all the user collection back.

Any help would be appreciated.