yes, have a fair idea about their research, but it's not a requirement as they tell you what they are/have been doing anyway. for example, if they are not your PoIs, know that dude A is working in field 1 whereas lady B is working in field 2 and similar. and that's usually enough because they end up explaining a lot more regarding what they do. you are arranged to meet with many people in the department because the department is looking for an overall fit between you and the program. if most/all profs agree that you can excel in their program, then you will be offered an admission. also, since it is likely that some of them will be in your future committee if you decide to go there, being professional and building bridges is what the other-than-PoIs interviews are all about. i met with 5 profs who were not my PoIs, and i had a great time... the review i got few days after the interview was really good. so, utilize the opportunity, and get to know some awesome people in your field!
congrats on the interview. am sure you will do great!