Do you have a large contact list in your Gmail? Is it hard to select the recipients? How many times you have missed important people when you send group email?
New Gmail lab feature will sugest you recipients while composing messages. This is what Gmail has to say about this new Lab feature.
"Once you've enabled it from the Labs tab under Settings, you'll see suggested recipients while composing messages. 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. Enter at least two recipients and any suggestions will show up like this:"
Official Gmail Blog: New in Labs: Suggest more recipients