As far as I know there is not possible to assign more than one assignee to a task. Have you ever considered this functionality? It could be set up at the workflow/category level whether the workflow/category can use more assignees or not.
It is clear that in theory only one person should be responsible for task. But quite often we face situations that one person is responsible for the task and more people are working on some smaller subtask in parallel. It can be solved by few workarounds like
- to use the assignee field for the responsible person and to define an user UDF for the "team" (BUT it makes notifications and filtering more complicated ...)
- to create separate subtasks for each team member (BUT it is quite an overhead since the subtasks are often tiny...)
