You will see the "Add to Calendar" link if:

• The invite was sent to a Google group for which the sender did not have permissions to view group membership (if you are managing the Google group, you can change this setting)

• The invite was sent to a Google group with more than 100 members

• The invite was sent from another calendaring system.

If you receive an event with an "Add to Calendar" link, be aware that updates to the original event will not automatically change the event on your calendar. In most cases, you will receive an update by email with another Add to Calendar link. You should add the new event and delete the old one from your calendar. The Yes/Maybe/No buttons below the Add to Calendar link will not work (they are just Gmail's interpretation of the .ics attachment that is on every calendar invitation). You will get an error message saying that email invitations cannot be forwarded.