Redmine

Different people are working together on one parent task, each sub task is assigned and visible to only one of them. But all sub task assignees should be allowed to view the (unassigned) parent task without having access to all tasks of the project.
The only way to achieve this is by creating a group for these sub taks users and granting access to this group for the parent task, but groups are not an option at this special requirement for some reasons.