Gmail Labs has a new feature that will appeal to those who frequently send messages to multiple people at a time. "Suggest more recipients" only works when you type at least two address in the "To" field and it suggests other contacts you should probably add, based on your previous usage.
"Gmail will suggest people you might want to include based on the groups of people you email most often. So if you always email your mom, dad, and sister together, and you start composing a message to your mom and dad, Gmail will suggest adding your sister," explains Gmail's blog.
While this feature is useful, Gmail could make it easier to add contacts when composing a message by linking to a contact picker, like the one that is included in many other Google services.