I think this is really a personal decision. I didn't bring any materials regarding my research, nor did anyone else on my interview day, but if it makes you more comfortable, go for it! I think if you feel like it would be an asset, you should bring it. It could also be something good to leave with your interviewers.
That said, I would be careful that you don't give the impression that you don't know your research back to front, or that you aren't comfortable answering questions about it. Again, it's really a matter of personal preference. I would have felt uncomfortable pulling out a poster, and likely would have looked to it to answer questions that I should know the answer to anyhow, but that's just me.
And the group activity is a lot of fun, and not very stressful. As other posters have said, it's just meant to simulate the PBL experience. You don't need any prior knowledge to make it work, so I wouldn't worry about it too much beforehand. You can't really prepare for it. Although the advice is trite, just be yourself!