Hi,
May i ask that how can i assign the task more than 1 person?
With the “assignment” mechanism, it’s not possible.
You can work out a different thing, maybe with Security Suite permissions. You can have the task visible to a group of people. It’s not the same thing, the tasks won’t show on the “my Tasks” filter (or dashlet) but you can easily create a view of the tasks the current user is allowed to view, and tell your users “if you can see it, it’s assigned to you”.
As a side-note: there’s a reason for single-person assignment: responsibility. You can blame the assigned user for not doing something, you can’t blame a group… 