![](http://3.bp.blogspot.com/_JE4qNpFW6Yk/SpSLzrgyNSI/AAAAAAAAAY8/yvFlRWasvVE/contact_chooser.png)
You can click on the contacts you want to add or search for others.
If you use contact groups, your groups will appear in a drop-down menu in the contact chooser, so you can select contacts from the groups you've already created. And if you happen to use Gmail in Chinese, Japanese or Korean, being able to pick from your list of contacts should be particularly useful since auto-complete doesn't offer the same search as you type experience that it does in other languages.
Official Gmail Blog: Composing a message? Try the contact chooser