Tasks' sharing features are under active development!
Currently lists can only be shared with other Tasks.org accounts. In the near future you will be able to share lists with anyone, even if they don't use Tasks!
Sharing features not supported due to technical limitations
Sharing features are available when your account is configured to use Tasks' native CalDAV synchronization
Sharing features will be available for native EteSync synchronization in a future update
Some list sharing features are available for OpenXchange servers. More features will be added in a future release if possible
Sharing support for other CalDAV services may be possible. If you're familiar with how sharing is implemented in your preferred CalDAV server, please submit a feature request with the details on Tasks' issue tracker