To invite more users to your account, click on Account in the left navigation bar (it's in the very bottom):
And choose Users from the drop down list:
Then click on Invite a user button:
And in the pop up window which will show up enter the new user e-mail address and determine their role (more about roles here).
Here's a short movie to sum up all the steps:
As soon as you invite the new user, they should receive an e-mail with a link to confirm the invitation.