It's confirmed the Guardians of the Galaxy - at least one member with Star-Lord - as well as the god of Thunder, Thor, will be appearing in The Avengers: Infinity War.
Joe Russo confirmed the news to the ComicBook.com web site.
The movies are intended to be a culmination of everything that's happened before in the MCU so you don't want to get into spoilers but I'm a big fan of what James Gunn has done. [Joe Russo and I] are both big fans of what James Gunn has done. Star-Lord is a fantastic character and Chris Pratt is an awesome performer so you'd be very excited.
I'm excited to work with Thor. They're doing some really interesting stuff with Ragnarok and where he's gonna be at the beginning of Infinity War is gonna be a very interesting place and I think very profound. I think he's gonna have a real emotional motivation after that.
It remains to be seen whether or not any of the other Guardians of the Galaxy characters make their way to Avengers: Infinity War. Interestingly enough, a while back a former Marvel employee that worked on the comic book told Cosmic Book News he/she expected the Guardians to be cannon fodder for Thanos.
And now we also know Thor survives Thor: Ragnarok.
The Avengers: Infinity War Part 1 has a May 4, 2018 release followed by Part 2 on May 3, 2019 directed by Joe and Anthony Russo.