
Enabling the feature works the same as all the Gmail Labs options—just head to the Labs tab under Settings and turn on the option. Once you've enabled it, you'll see suggestions once you've added at least two recipients to an email message. It's a simple but potentially great time-saving feature—just one of many Labs features you should enable.
For more, make sure to check out the 10 Gmail Labs features you should enable, and then check out the Best Gmail Labs features and user scripts.
New in Labs: Suggest more recipients [Official Gmail Blog]