The filmmakers behind 2018's Avengers: Infinity War probably aren't ready to talk too much about its secrets, but Vin Diesel doesn't give a damn. On Facebook Live, the voice of Groot revealed that, yes, he and the Guardians of the Galaxy will be in the next Avengers film. Speaking on Facebook, Diesel said the following:
....the Guardians will be included in Avengers: Infinity War and that's incredibly exciting. Incredibly exciting. Incredibly exciting and something that if you were a part of our page you knew before everybody.
Obviously, this is something we've all assumed for a long time, but to have one of the actors flat out say it is pretty surprising.
What I'm now wondering is how the Guardians become involved. Guardians of the Galaxy Vol. 2 director James Gunn has stated multiple times that neither Thanos nor any Infinity Stones will appear in his 2017 film. So my guess is Thanos goes after the Stones in Infinity War, which sets him up for a showdown with the Guardians.
This is important, though: What Diesel didn't say is if the Guardians will appear alongside the Avengers. They could very, very easily just be in a tangent of the movie set and not make their way back to Earth to mix it up with Captain America and Iron Man. Or, maybe the Avengers make their way into space thanks to Thor and Hulk and run into Rocket and Groot. All of this speculation is why these next to Avengers sequels are so freaking exciting to talk about.
Avengers: Infinity War, featuring the Guardians of the Galaxy, opens 4 May 2018 in the US. Its still-untitled sequel will open a year after that.