If you delete the Multi-User Columns view, you can create a new one:
Click Calendar in the Nav Bar.
Right-click the Calendar toolbar, and then click Properties.
Click New, and then specify a name for the Multi-User Columns view, such as Multi-User Columns.
Click Multi-User (Columns) in the View drop-down list, and then click OK twice.
The Multi-User List dialog box displays.
If a user or resource you want in the Multi-User Columns view is not listed, specify the user in the User Name field, and then click Add User.
Double-click a user or resource, click OK, and then repeat for each user you want to add.
Select the check boxes of the users whose calendars you want to display, and then click OK.