add user in grouptask error

System

In TeamSite 5.5.2, it shows an error "users xxxx is not allowed to add users to this task" if a workflow has a grouptask like below

<sharedby>
<user v="alex" /><user v="ben" /><user v="calvin" />
</sharedby>

However, if the workflow owner is changed from master to job initiator by owner="__TAG__('iw_user');", it works normally.

Does anyone know the reason and suggest a way to use grouptask while the workflow owner is kept by a user in master role?