This just came up on my Facebook feed, a developer/fan has made it possible to text with Groot. The number is 866-740-4531 and it works, I just tried it out.

ETA: 866 is supposed to be a toll free number, but if you have texting limits that doesn't really matter.

From Tech Crunch via

And here's a Tweet from the creator, after being validated by James Gunn.