Unsure about your specific program's qualifications, but I have been to an interview like the one you explained. Basically a group of maybe 10/15 of us were in a session together. The committee members will ask you all questions like "how would you approach (_____) situation?" "What are your thoughts on XYZ" etc. Basically- yes, a discussion group. I was under the impression that the committee was trying to get an idea of what we would be like in a group settings, and trying to gauge our current knowledge/investment in topics in psychology. Lots of ethics-type questions. Just be yourself, and make sure to not let the group setting intimidate you. Make sure to complement other's responses and to be personable. I actually liked this format better because it was less of me trying to sell myself, and more just talking on the topics I am passionate about.
Best of luck to you!