I love using Google Calender and the Calender Widget on the phone. I never miss appointments or other important events anymore. I use Google Sync to sync my work Outlook with Google Calender. https://tools.google.com/dlpage/gappssync
. I also use Mozilla Sunbird
on my home laptop to add and view events. You just need the Google Calender plug-in to get it to work with Sunbird.
Now for your question, you can add attendees through the Google Calender website on you phone. It's pretty simple to use. It would be nice to be able to do it through the widget.