It seems that API allows to assign users to a task (called an Item in the API) using the body field responsible_uid at the add an item endpoint. However I cannot find a way to list user uids or any other way to get user details anywhere in API documentation.
Official python library todoist-python doesn't provide any way to do this either. So for now it seems I can only create tasks without assigning them to anybody, which is a bummer.
Any advise grately appreciated!
Links:
You should first share a project and then you can get all collaborators in the Sync request.