Based on descriptions on the FHI website, it looks like Kyle Scott filled this role, from July 2015 to September 2017.
From the earliest snapshot of his FHI bio page:
Kyle brings over 5 years of operations experience to the Future of Humanity Instutute. He keeps daily operations running smoothly, and manages incoming and outgoing requests for Prof. Nick Bostrom.
Strategically, he works to improve the processes and capacity of the office and free up the attention and time of Prof. Nick Bostrom.
Kyle came to the Future of Humanity Institute from the Effective Altruism movement, determining that this job position would be his most effective contribution to society. Learn more about Effective Altruism here.
The page is still up but it doesn't look like he holds the position anymore.
He seems to be a project manager at BERI now:
Kyle manages various projects supporting BERI's partner institutions. He graduated Whitman College with a B.A. in Philosophy. He spent two years working in career services and subsequently moved to Oxford where he worked for 80,000 Hours, the Centre for Effective Altruism and most recently at the Future of Humanity Institute as Nick Bostrom's Executive Assistant.
On November 13, 2017 FHI opened the position for applications.
ETA: Louis Francini comes to the same conclusion on Quora. (Context: I asked the question on Quora, figured out the answer, posted this comment, then Louis answered my question.)
If funding were available, the Centre for Effective Altruism would consider hiring someone to work closely with Prof Nick Bostrom to provide anything and everything he needs to be more productive. Bostrom is obviously the Director of the Future of Humanity Institute at Oxford University, and author of Superintelligence, the best guide yet to the possible risks posed by artificial intelligence.
Nobody has yet confirmed they will fund this role, but we are nevertheless interested in getting expressions of interest from suitable candidates.
The list of required characteristics is hefty, and the position would be a challenging one:
The research Bostrom can do is unique; to my knowledge we don't have anyone who has made such significant strides clarifying the biggest risks facing humanity as a whole. As a result, helping increase Bostrom's output by say, 20%, would be a major contribution. This person's work would also help the rest of the Future of Humanity Institute run smoothly.