This is the best way to learn! 🙂

Ok so my preferred way to return multiple values is to place them into an object then stringify (convert the object to a string) and return to the client script. The client script can then convert back to an object and you can do with the values as you please. For example taking your script above (getUserName) lets say your wanted to return multiple values from the user record for use in the client script it would look like this: p.s ive changed the name of the function to getUserDetails

var GetUserInfo = Class.create();
GetUserInfo.prototype = Object.extendsObject(global.AbstractAjaxProcessor, {
	
	
	getUserDetails: function(){
                var obj = {};		
		var userRecord = new GlideRecord("sys_user");
		if(userRecord.get(this.getParameter('sysparm_userID'))){
                     
                  
                  obj.first_name = userRecord.getValue('first_name');
                  obj.last_name = userRecord.getValue('last_name');
                  obj.email = userRecord.getValue('email');
                  //Reference Field will return sys_id. Use getDisplayValue() to return display value
                  obj.manager = userRecord.getValue('manager');

                }
		
                 return JSON.stringify(obj);         
	},


    type: 'GetUserInfo'
});

Then to consume the multiple returned values you can parse the returned object string:

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
 
	if (isLoading || newValue === '' || newValue == oldValue) {
		return;
	}
	
	var getUserIn = new GlideAjax('GetUserInfo');
	getUserIn.addParam('sysparm_name','getUserDetails');
	getUserIn.addParam('sysparm_userID', g_form.getValue('sn_userid'));
	getUserIn.getXML(populateUserDetails);
	
	
	function populateUserDetails(response){
		var answer = response.responseXML.documentElement.getAttribute("answer");
		var obj = JSON.parse(answer);

                g_form.clearValue('user_firstname');
		g_form.setValue('user_firstname', obj.first_name);

                g_form.clearValue('user_lastname');
		g_form.setValue('user_lastname', obj.last_name);

                g_form.clearValue('user_email');
		g_form.setValue('user_email', obj.email);

                //Assuming this is a reference field to sys_user table which will consume returned manager sys_id
                g_form.clearValue('user_manager');
		g_form.setValue('user_manager', obj.manager);
	}
	
}

Hopefully this helps a bit.