Specifically I am trying to find a work-around this problem, also posted by another user on Trackplus sourceforge forum
"On a given project, we want the manager to assign a task (say to person "toto"). For this, it seems that toto MUST be a responsible, otherwize how can the manager assign toto the task (the "responsibles" popup only shows ... responsibles). I also don't want toto to see other peoples' issues, but only his own issues and be able to modify them. But being a responsible gives him the power to see all tasks"