I've figured this out using a combination of GetUser, Mid, and Split functions. The GetUser returns the long string of id, username, name, email, etc., so I used the Mid function to separate a portion of it, where the username begins. Since our all of our users usernames are different lengths but have a "@" symbol in them, I was able to use the Split function to split it by the "@", which returns an array, so then I called to return the first part of the split array (array[0]). I only need this first part in my data. See below for what I entered in the calculate field tool. I'm sure there's an easier way to do this.
var userinfo = Mid(GetUser(Portal('https://www.arcgis.com')),53,20);
var array = Split(userinfo,"@",1);
var username = array[0];
return username;