A Rachio owner can grant access to anyone with an e-mail address. This provides two levels of operation: limited, basically allowing someone to check if anything is running and to initiate running a zone; and full, giving access to the entire UI,
If someone has basic web development skills they can integrate using Rachio's REST API. The REST API provides access to many Rachio features.
IFTTT can be used to create custom integrations, and there are many integrations described in the knowledge base http://support.rachio.com/category/438-integrations