From your dashboard menu, select the "Users" tab. On the next screen, click the blue "Add user" button. Fill out the popup form with the user's information (the device key should auto-populate) and click "Save." When the screen refreshes, you'll see the user listed in the table, alongside their device key and the rest of the form information.
If you see an error message when you click "Add User," you may have User placeholders in your account (in the format "User 1" or just the number "1"). In that case, on the "User Management" screen, click the pencil in the row next to a user. In the popup form, simply change the user details and click "Save" when you're done.