In the novelization, Draven said "Squadron up" to a Captain Vienaris.
Later, it was Private Weems (the communication technician who later rushed to Mothma telling her there were Rebels on Scarif) who said "They are already engaged".
Draven swore inwardly and gestured rapidly at Weems. “Get the squadron leader on,” he said. “Get him on now!”
Weems looked as aghast as if he’d been accused of desertion. “They sent word three minutes ago. They’re already engaged, sir.”
There is no Captain Vienaris in Rogue One the Ultimate Visual Guide, or in credits. The fact that the two notable actions/interactions this character did in the movie is attributed to two different characters in the novel makes me think that this character was unplanned.
I was itching to know who he is as well! The above may not be an answer (I wanted to add in a comment). I can't believe they overlooked a character as prominent as this guy, a character with spoken lines, when they credit likes of "Jedha server 1".