The Users page shows the list of users, as well as giving you some actions which you can perform use the user list.
Just above the main user list is an entry Outgoing Message Queue. Clicking on this will let you see which messages are waiting to be sent out, and you can delete/hold the messages in there.
To edit a user, double-click on the user in the user list. To add a user, press the New button. To delete user(s), select the user(s) and press the Delete button.
The user list has several columns:
Users have the following tabs in their settings window: