Using Mashup with Google forms we have been able to create a guest invitation, and acceptance features which Sumeet Jindal requested. A set of questions can be created in Google Forms, linked to a google spreadsheet, and emailed to all the invitees. The invitees can see the content and fill it up as well inside their emails. All the response will come back to Google Spreadsheet. This accomplishes acceptable level of guest invitation feature.
There are 2 problems which we will solve when we are not cash constaints.
a. Google Forms does not have god validation features. So junk responses get in the spreadsheet. A custom form with needed validations can be created to sort this out
b. Google Spreadsheet loads slow: Google docs provides API to directly access spreadsheet data without loading spreadsheet itself. An application with those APIs can be created. This will be done later