From Help file:
The user or group to retrieve data from. This task can only be assigned to a single user or group. If it is assigned to a group, the first group member to respond will act on behalf of the entire group.
Note: Although the People Picker field will allow multiple users to be added, the first person added will be the only person notified and the only person who can approve the task. If a group is added, all will receive the notification, but only one person will be able to respond. This is because the workflow action will only create a single task, and only one user will be able to respond to the task. Also, once one person responds to the task, the task will be completed and the other user will not be able to provide input.
Thanks for your reply. Actually I have a column in list i.e, "Task Owner" in that I have checked "Allow multiple selections".
When I assigned request data action to "Task owner", all users get notification, but nobody have permission to work on the action.
On task page, message displays "You are not authorized".
Please let me know, is there any way to create a group and add all "Task Owner" user in that group then assign Request data to that created group (Using Nintex workflow)