Before interviewing at a grad school, how much background research should I do into its research groups?
My experience interviewing was in the US, so it might or might not apply to you in Austria.

None of the schools where I interviewed expected me to know the details of their latest work. I read a few of their recent papers so I had some idea what the groups did and I could have an intelligent conversation with professors and other students. It seemed to go well.

Good luck!