If you are in normal user mode, as opposed to administration mode, you will see a Directory link available instead of the Users and Groups links.
Clicking on Directory, you'll see a screen like this:
From here, you can see a list of all the users on the system, as well as their full names and phone numbers, if provided. In a column called Action, there are links that allow you to e-mail a particular user, or to visit that user's personal web page, if it exists.
Below that, there is a listing of all the user groups on the system. You can e-mail all the users in a particular group by clicking the word Email next to the group name.