Next to each mailbox, you will see a settings button with the following options to manage the box:
Manage: This allows you to change the mailbox password.
Control Panel: This gives access to features for that user. The user may also access this in their webmail. This is most helpful for tracking emails sent by that user, checking for spam, etc.
You can Enable, Disable and Delete accounts from this menu as well.
Webmail
Click to access that user's webmail. This is specific to administrators… the owner of the main account. See How do my staff log in to the web-based email?