After speaking with the wrike team, it appear that it is not possible to assign rights on a task base. It woould be pretty nice to be able to assign rights to a person only for a certain task. It would allow to work on some task without having to share the entire project with unrealevant people.
In addition to that, it would also be nice to be able to set the default rights when doing some action (by example, if you mention someone on a task, you could set the default rights to be "Limited" on the task).
What do you think about it? Would it be possible to add these functionalities?