Click on "Firm Administration" in the menu that appears below your name in the top right corner, then click the "Users" tab.

You’ll see an “Invite User” button to invite other users associated with your firm.  Note the check box that allows you to make the new users a firm administrator, as well. When you click to invite the new user, an email will be sent telling the new user how to activate their account.  Keep in mind that all of your firm administrators have the ability to edit your firm's profile, so be selective in adding new users. For the same reason, be vigilant and make certain that if one of your firm administrators leaves your firm, you immediately remove them from your approved administrators list.