As of Salesforce’s summer 2012 release, you can now add multiple contacts to a task using the shared activities feature (which you must enable in setup).
This feature should help you more efficiently use tasks and log activities. For example, say you want to create a task for a meeting with a large number of people. Rather than having to create the same activity separately for each contact, you only need to do it once. Then add as many contacts to it as needed.
Implementing the multiple contacts feature for events and for emails (Email to Salesforce and My Unresolved Items) is in Salesforce’s development pipeline for future releases.
Source: Kayvaan Ghassemieh, Salesforce.com Product Management