While reading the description of the film, I noticed one thing: they didn't write just Avengers, they wrote Avengers and Guardians of the Galaxy.
I wonder if Guardians of the Galaxy are part of the Avengers.
In the MCU, the Avengers and the Guardians of the Galaxy are two separate and independent groups of heroes.
In the comics, individual Guardians have, I believe, been included in the roster of Avengers but that could probably be said of pretty much any Marvel hero by now.
There have been many, many roster changes over the years.