Here is tesla's comment on it, but it seems like they will be fully in control of the fleet/app software and could potentially favor their own cars in cities where they are running their own fleets over your own cars you have added into the fleet as it benefits them financially more.
Potentially "jailbreaking" and building your own fleet/app would be an option, not sure how legal but if tesla issues a critical security update and the vulnerability gets published by the researcher and the the jailbroken ridesharing provider you are using hasn't updated yet/can't because it will break their jailbreak in the short term, you are exposed
"When true self-driving is approved by regulators, it will mean that you will be able to summon your Tesla from pretty much anywhere. Once it picks you up, you will be able to sleep, read or do anything else enroute to your destination.
You will also be able to add your car to the Tesla shared fleet just by tapping a button on the Tesla phone app and have it generate income for you while you're at work or on vacation, significantly offsetting and at times potentially exceeding the monthly loan or lease cost. This dramatically lowers the true cost of ownership to the point where almost anyone could own a Tesla. Since most cars are only in use by their owner for 5% to 10% of the day, the fundamental economic utility of a true self-driving car is likely to be several times that of a car which is not.
In cities where demand exceeds the supply of customer-owned cars, Tesla will operate its own fleet, ensuring you can always hail a ride from us no matter where you are."
https://www.tesla.com/blog/master-plan-part-deux