The other day someone asked me how a manager would be able to tell that a candidate for a business analyst job has communication skills. Here is my answer:
Instead of engaging in the apparently mandatory Q&A session where the applicant answers fairly rote questions (so rote that there are companies and people who purport to be able to prepare you for the questions you will get in an interview), start a conversation with the applicant. Ask questions that you might ask someone on first meeting them at a party or social get together.
The answers are unimportant. You are looking for the way that they answer to determine their communications ability and skill. If the answers are monosyllabic, evasive (a good communicator is willing to state that a particular subject is off limits or they do not wish to talk about it, rather than simply evading the question or providing ambiguous answers), not to the point, or answering in such a way that you are confused about their answer or not answering at all, then the person is demonstrating a lack of communication skills. On the other hand, if the applicant grabs the questions and runs with them, dominating the conversation (and as a good interviewer, you let the behavior run its course) then the applicant is also showing a lack of communication skills. The questions should not be too personal or threatening (hence the cocktail party analogy). Develop some questions that would typically evoke questions in response. Listen to how the applicant asks the questions (if the applicant does) and listens to the answers. Again this is a good indication of communication skills. A trick one friend of mine uses is to start the interview with some information about himself or the company and then ask a question later that would tell him whether the applicant was listening well.
Some experts point out that the interview situation is fraught with stress and that generally applicants in that pressure situation will revert to short answers. Ask more open ended questions as a way of getting the applicant more relaxed and responsive. However, if the applicant cannot get over their reticence in the interview they likely will not be able to overcome it when they have to interview a Vice President about a new project.
I would refrain from doing group interviews with the whole team. The concept there is to see how the applicant will fit in with the team. However, such an interview is terribly unrealistic. Unless you are hiring a person who will be making a lot of formal presentations to upper level management and other judgmental panels, This is not common. A better approach is one-on-one interviews with the team. This approach also has the advantage of some OJT training in interviewing for the team members.
As for the skills, my assumption from your post is that the technical skills or experiential skills have been handled through resume screening or a screening interview by HR or something so everyone you might talk to is technically qualified and you are looking for those with the right set of soft skills for a business analyst.