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.

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