Hi there.
See this http://share.ez.no/forums/install-configuration/event-finish-user-account-workflow-event-issue/ for reference.

there seems to be a mismatch between doc a real functionality provided with eZ publish. reading doc it says

Select "Users" under "Affected Sections" and "Publishing New Object" under "Affected Versions".
Select proper group who can approve the event 
Press "Add groups" under "Users who approve content" and select the group you wish to give control of approving the new users.
Press "OK" to save. 

This four steps are not possible. template provided for the workflow doesn't add forms to select users nor groups and the php class doesn't have the functions for getting the values.

https://github.com/ezsystems/ezpublish/blob/master/design/standard/templates/workflow/eventtype/edit/event_ezfinishuserregister.tpl

https://github.com/ezsystems/ezpublish/blob/master/kernel/classes/workflowtypes/event/ezfinishuserregister/ezfinishuserregistertype.php

Cheers