How to know the id of the user who picked up the task and to know the id of your organization?

I'm having a hard time running a certain task in my process, my need is the following, I need the user to start certain process in the portal, this task will be automatically assigned to it, so do not need to get the terefa again when it opens in Your task list, however, what I am having difficulty is that if the user resolves to release this task, it may appear again to all users enabled to perform this task, however, as I am choosing the user ID that Is in Session at the time the process starts and I'm applying an actor filter with this id, this task always appears only to the user who started the process. I can not delete this filter when the user re-launches this task again.

Is it possible to do that? Zeroing the actor filter when the user releases the task, any suggestions will be accepted.

Thank you