As a administrator, if you click on your name in the upper right hand corner and then click "Manage Account," you will be brought to the Manage Account screen where you will find a list of all invited and added Members on the account.
Active Members must first be deactivated and then can be deleted using the "Delete Member" link/button.
If a Member is already inactive (for example if they've been invited but never created their own login), they can be deleted in one step by clicking the "Delete Member" link.