Assign task to the group or perticular user through interface.

There are two ways
1. You need to use Appian out of the box feature in the USER INPUT TASK configuration in model

2. You need to create your own UI, where you need to pull all the task which you want reassign and on selecting those task, show a picker/dropdown with recipients as value.  On submit call a process model and use a smart service of reassign.

Below is the plugin which will give Reassign + extra task management smart services

community.appian.com/.../task-management-smart-services