Sometimes things change and you need to quickly contact your customers to keep them up to date.
To 'email all' customers you need to have your emails setup locally on your machine, once this is ready to go follow the steps below:
1) Go to Calendar and select the activity
2) Click 'Manage' and 'Email all'
This will load your local email program and create a new message with all of the organisers emails blind copied (bcc) ready to go. By using your email system this ensures that all replies come directly to you and not via eola.