A task can be successfully redirected to a group using the WorkflowManagementServer.RedirectWorklistItem() API, however the group's members do not see this task item in their worklist.
This is currently expected behavior. It is currently not possible to redirect/delegate a task item to another group/role; only a user will work.
Although usage of the WorkflowManagementServer.RedirectWorklistItem(fromUserName, toUserName, procInstID, actInstDestID, worklistItemID) method will allow the task to be redirected to any sort of string value and the Management Worklist will show it being assigned to this string; the method is expecting that this string is a valid user in the K2FQN format (K2:DOMAIN\username) or (SecurityLabelName:username) for the specified user to be able to see the task item.
This is similar to the OOTB Redirect behavior seen for the SharePoint Worklist Webpart, K2 Workspace worklist, or K2 SmartForms Worklist control; such that only a user can be selected.
As the RedirectWorklistItem() method is expecting a user object, you will need to validate that the string value is a valid user object prior to passing it into the method.
An idea can also be logged/upvoted at (https://ideas.k2.com/) to possibly support redirecting to a group/role in the future.