Avengers 4: Every Character Confirmed So Far
3. Nebula
Confirmed By: The Avengers Facebook
Again, Zoe Saldana confirmed all of the Guardians, but Nebula isn't really an Avengers so that might not have covered her. More firm proof came with the Red Cross video for Hurricane Harvey that was posted on the Avengers official page, showing Nebula.
And very interestingly, it looks like they might at least partly be appearing in a flashback sequence, as the video shows them wearing their costumes from Guardians Of The Galaxy Vol 1. Time travel anyone?
Even without that, her survival makes her a guaranteed participant.