It works as if you're in the US.
You might need to make sure their phone number is in the format of +1-404-555-1212 (need the +1 -- the hyphens are just grouping for me, you can use parentheses, or spaces or nothing. It just needs +14045551212).
The recipient will not get charged anything for receiving a text (unless their plan says they get charged for receiving any texts). It's coming from a US number. They don't care where you are. It's the sender's responsibility to get it from them to the recipient.
If they text you back, they only get charged whatever their carrier charges to send a text to the US, as they are texting a US number. It's not their responsibility to know if you're in the US, Canada, France, or Saturn. Once the text gets to the recipient's carrier, it's the recipient's duty to pay any extra fees to get it to wherever in the universe they are. In this case, T-Mobile includes texting, so you're fine. No additional fees.
In short, no fees for either side, assuming both sides have texting on their plans.
The same concept above goes for calling as well.
If your neighbor back home called you and you answered the phone, they won't get charged. You'll get charged whatever roaming calling rate you have on your plan (I think it's 20c/min, but don't quote me).