a user of my app wants to access tasks shared with him via the app. Apparently, they are not visible via the Api, right? Is there a way to access shared tasks or is this a feature available only to the toodledo web site?
Shared tasks are not yet available via the API. You could provide a way for your app to authenticate with multiple accounts and fake it, but it would require that a person's collaborator enter their password into the other person's device, which probably isn't a great idea.