Head to the People section from your left navigation menu. Then click the green "+ New user" button in the upper-left part of the page:
Add their name and email address (but note that name cannot include digits, so any numbers will need to be typed out).
Next, set their User Access Level. Choose between Limited, Normal and Admin.
You can read about limiting a user's access level here.
Then set their hourly project rate. If the user has a flat rate, you can choose to use the same rate for all projects.
You can add Users to as many teams as you like. Just search by name or use the drop-down menu.
Finally, choose the projects they're part of!
NOTE: if the Project uses the same rates for all users, the pre-set project rate will override individually set user rates.