Part of the beauty of Google Apps is the ability to communicate with lots of people at the same time. If you regularly email the same people at the same time, try setting up a group that includes all of their email addresses. That way when you fill in the "To:" field of your email, you can just type the group name and it will automatically add all of their email addresses. Here's how:
You can set up as many groups as you want to, and you can add the same person to multiple groups. That way all of your contacts stay organized the way you want them to!