Update: Sign up for this offer ends tonight. Use this link to sign up.
They will also offer the MLB.com At Bat Premium app for free, a $19.99 value.
Once you’re registered for MLB.TV you can use the account on multiple devices simultaneously.
Note that streaming games from non T-Mobile phones can lead to the free subscription being terminated, though you can signup from a T-Mobile phone and then stream from non-phone devices.