To add a new user to your account from the left menu please go to settings -> account configuration.
At the right side of your screen, find the user accounts box and choose the "+" button.
Type the email that you need to invite in the first field, choose the access of the user (admin, editor, or viewer) and send the invitation!
You can check the state of a user if he/she has accepted the invitation or not. If a user has not yet signed in to the account in the notes the state will be "needs validation".
You can simply remove a user permanently from your account by clicking on the "-" button!
Try the pencil button to make any changes, eg change a user's access from viewer to editor.