There are characters from movies not even out yet who are being added to Avengers: Infinity War. At this point, if you are in the MCU and you haven't been asked to show up in Infinity War, I would really worry.
Pom Klementieff, who plays Mantis in Guardians of the Galaxy Vol. 2, announced she would be in the new Avenger flick with some pictures posted to Instagram. A few days back, she teased with this one:
And then, yesterday, she apparently confirmed her appearance with this one:
And so the antennae-d telepath joins Thanos, Iron Man, the Hulk, Captain America, Thor, Hawkeye, Star-Lord, Gamora, Groot, Drax, Scarlet Witch, Winter Soldier, Doctor Strange, Vision, Maria Hill, Nick Fury, Wong, Nebula, Loki, and, presumably, the kitchen sink as one of the characters making an appearance in Infinity War.