This button is not enabled when in edit mode. When enabled and pressed, it will display a series of prompts in order to make a copy of the currently selected user account and displays the following warning:

The following prompt requests the new cloned user name:

The entered name must be unique i.e. different from any existing user login names.
The following prompt must be accepted (Yes) in order to create the new user account:

Another prompt requests that an existing contact is chosen as the contact user (only for legacy installations) (Yes), or that a new user contact record be chosen:

When choosing No, the following prompt requests that the existing company should be chosen (No):

The following dialogue is shown to capture the new user contact details:

When the user contact record has been created, the new user is added to the grid and the contact user details updated:
