There are several places that members can indicate their availability for an event...
1. On the event's page
Visit the event's page, and then look at the section called Attendance/Availability:
2. On the Upcoming Events page
Note that after selecting any options on that page, the member must click the Update Attendancesbutton at the bottom of the list.
3. On the main Members page

4. From an Event Notification Email
This is probably the most useful option, as all members are notified about the event via an email, and can indicate their availability with one click.
To send out the event notification emails, click the Email event now link at the top of the event's page:
Follow the prompts, and a personalised email will be sent to each member. When they receive the email, they simply click the appropriate link in the email to indicate their availability: