I found out about the open position through a friend/former colleague that works at Glassdoor. Once I sent her my resume, the process was very quick (around two weeks). I had two hour-long phone interviews with two members of the PR team, and then was brought in for four more in-person interviews with the PR team and a co-founder. Before the in-person interviews, I was also asked to write a press release based on data points from the site, and fill out an "ideas" document.
If you're going to interview at Glassdoor, think like a Boy Scout: Be Prepared. You will be asked to explain your press release and why you picked that angle. You will also be asked to pitch your press release to your interviewer (he stepped out of the room and had me pitch him on his cell phone). You will also be asked about your media contacts, your recent PR wins, and some of your favorite ideas from the document you filled out. Also be prepared to answer a few more "out of the box" questions like... explain Glassdoor in three words, pitch me the Glassdoor app but pretend we're in a noisy bar, pitch me Glassdoor to the least tech-savvy person you know. Like I said - Be Prepared!
From what I can tell, everyone at Glassdoor is hardworking and loves working hard at Glassdoor. They want to make sure that anyone they hire would fit into this culture. If you're not willing to really do your research and think about what you could bring to your team, then it's likely not a right fit for you.
This was the hardest interview I've ever had, but the most rewarding interview I've ever had. I learned just as much about Glassdoor and my future team members as much as they learned from me. I walked away from the interview knowing that accepting their offer was 100% the right decision for me.