How do I send an IM to more than one person?
To IM more than one person at a time:

First open up your Yahoo Messenger contact list. Then hold down the "Ctrl" key on your keyboard and click on the contacts that you want to message. Click the "Actions" menu in Yahoo! Messenger and select "Send an Instant Message." Now just simply type your message and send.

To send an IM to a Group:

Right-click the Group and then select "Send Instant Message to All in Group." Now type your message and click the "Send" button.

Right click group name and select "Broadcast IM to this group" in Yahoo! Messenger 9.0 beta.