Go to Settings - Account
Scroll down to the bottom of the page until you see the Additional User Profiles section
Click on the plus sign:
A popup will open where you need to fill in the new user's First Name, Last Name, email and select a user role which will define the level of access that will be provided to this account. Read about different user access levels HERE.
Click on Save.
That's it - a new user has been added and the email is sent to his new account with further setup instructions.