Think that a workflow runs on background, so when a workflow action is executed there is no a logged user.
Perhaps you want to know the user that completes a task, in that case you can use the one named "Last task respondent", so with that you will have the username (domain\username) of the user who last registered a response to a task.
Nintex Workflows usually run under the context of the initiator so it has no concept of grabbing another logged in user once it is already started. You might be able to use a list whereby a user modified a field that then allows the workflow to realize this is the person is should act upon in some way next. Fernando also made good suggestions above.