Hi Virk,
You can use the todo Plugin
https://www.dokuwiki.org/plugin:todo
It should work in the newest version of DokuWiki 2013-03-06 "Weatherwax".
Your additional functionality "show user which completed the task" is on the implementation list for the next release.
At the moment you can edit the todo and assign the user by using the @ symbol. example:
<todo @leo>Task for leo</todo>
<todo @leo #>Completed Task for leo</todo>
I also integrated the new searchpattern Plugin
https://www.dokuwiki.org/plugin:searchpattern version to show all todos from all sub pages on a single page.
A documentation how to configure this is available at
http://www.eibler.at/dokuwiki-todo/index_en.php
regards,
Leo