Q: Where can I specify the partners who are active in a task or as the Task Leader?

The Task Leader can only be set by the Project Coordinator through Proposal > Work Plan > WP Structure. As for partners being assigned to tasks, go via the claiming plan of person-months under Proposal > Work Plan > Resources.

PreviousQ: Is it possible to add plan person-months without adding the costs? NextQ: Can I modify the project plan duration at the Proposal Stage?