Click on the section called People on the navigation menu on the left of your screen. In the upper left part of the page you'll see a green "+ new user" - click on it.
Add their name and email address. Please note that the name cannot include digits; numbers will need to be typed out.
Next, set their User Access Level. Choose between Limited, Normal, Admin. Read more about limiting a user's access level.
Set their hourly rate per project. If the user has a flat rate, you can choose to use the same rate for all projects.
And finally, you'll need to decided what projects they are added to.