Try this:

var arguments = "/c net localgroup " + group + " " + userCode + " /add";

Basically, just add /c

http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/cmd.mspx?mfr=true

You can test this whilst logged in by setting interactive session to true, logging in with the creds used in the work flow.