How do you create a new desk top on a mac computer?

Go to SYSTEM PREFERENCES Click on "Expose' & Spaces"

Click on "Enable Spaces"

Add the number of Rows/Columns of Desktops you would like.

IF you're talking about new USERS,

Go to SYSTEM PREFERENCES

Click on "Accounts"

Click on the little + sign under "MY ACCOUNTS" and add users at you see fit.