Organic groups RSVP adds attendance list functionality to event on a group-level only. It functions simply by taking members of groups selected while adding or updating an Event node and adding them to an attendance list (also optionally sends an e-mail).
Event nodes with attendance lists contain an easy to use interface for members to select their attendance status (Attending or Not Attending). The status of all members from invited groups are displayed above the attendance status form.
When a user is removed or added to a group they are invited to all future events that group is currently an audience of. Additionally when a group is removed or added as an audience the users are removed or added to the attendance list.
The Event Views module is listed as a dependency despite not being officially released for Drupal 6. Please see the issue queue for more information.
Note: This module is mainly deprecated in favor of rsvp. A bit after I created the project, I wrote a patch for rsvp 1.x for og support. Now, in rsvp 2.x, og support is standard and it supports both event and calendar implementations.
OG RSVP was originally developed for a specific calendar project by Kosada, Incorporated.