Adds people to a phone bank, text bank, or work list (any task whose contact_strategy is one_to_one) from a filter, a saved user list, or explicit user ids. Send exactly one source. Only eligible people are added (active, phone number, contact permission for the task action) and people already on the task are skipped. Assignment follows the task assignment_strategy and its team members (POST /task_agents). With persistent true the filter is saved on the task and re-run every 30 minutes to add new matches. With max, the request is refused (422 with matched_count) when more people match than max and nobody is added. Up to 500 new people are added inline (201); above that the work is queued (202). For one person at a time use POST /task_assignments. Work list tasks require the Work Lists feature on the billing plan.