Main role: Be responsible for the poster/demo presentations during the conference. This includes:
Direct the poster/demo paper reviewing and selection process
Make the poster/demo program
Run the poster/demo sessions during the conference
Prepare the proceedings for archival
Clarification of the role of poster/demo chairs
In many conferences there have not been separate poster/demo chairs, as this role has been covered by the Paper Chair. In other conferences there have been dedicated poster/demo chairs, who have been working with the paper chair during the reviewing and selection process. The main difference is probably during the conference, where the paper chair is typically responsible for running the oral presentations, while the poster/demo chairs run the poster/demo sessions.
Clarification of the role of demo papers
There is some confusion about what a "demo" is in the NIME context. This is because historically there have been two uses of the term:
Demo papers (2 pages in the proceedings). These have been submitted alongside long and short papers. They have been peer-reviewed as part of the regular paper process. They have been shown at the conference alongside posters. Therefore many people have not understood the difference between posters and demos. The demo papers have also gone into the paper proceedings, although there has been some discussion about this practice. Since they are only 2 pages long, many demo papers are difficult to peer-review properly. Some have thought of them as similar to a "late-breaking demo" and that they should not be included in the proceedings. Others believe that the demo papers are a good way of opening the conference for people, particularly those coming from more practice-based disciplines.
Demos of "things" from oral presentations. What is confusing to many is that oral presenters have often been encouraged to also demo their "thing" in one of the poster/demo sessions. This has been a way of letting people that have talked about a cool instrument also showcase it in a demo setting. These demos have not been submitted separately, they have been "invited" based on being selected as either long/short papers. So they are scheduled in the program, and will appear in the proceedings as the short/long paper that they were originally selected from.
There has not been an official non-paper demo track at NIME, although several conferences have experimented with various types of "NIME Showcase", "NIME Show and Tell", or "NIME Late-breaking Demo" events. This has been a way of presenting the work of local students and practitioners. What is important, however, is that presenting at NIME in such a way would not mean ending up in the official paper proceedings. Such "showcase" presentations have not been peer-reviewed, but rather curated, selected, or invited. This is an important contribution to the conference, but should not be confused with getting NIME "publication points".