Before we start, it's important to note you must be an Owner or Admin on your team in order to invite people.
User Settings Screen
Simply click the team drop down at the top right of the screen and click Settings. From there click Users which will bring you to the Team User Settings screen. (Shown below)
Invite a user
To add a user, simply type in their email and hit the invite button to the right. If you want to give them admin access, simply check the admin button before clicking invite.