Yes, one Task can have only Responsible person.

But it's possible to "assign" several people to the same Task using the record "Work on Task" option: this adds to Tasks a grid to record time on tasks, and each line can correspond to a different person.

It's meant to record time spent on tasks, but you could also use it to plan people assigned on Tasks: just leave the Time/Hours spent as zero when assigning people to it. Later on you can edit them to record the real time spent. if you wish.