Personally speaking, the biggest incentive that would motivate me would be if the interest group was based on an issue that I felt very strongly about and also could conceivably accomplish some form of change or impact in the world. I don't think that there would be any other incentives necessary apart from these two factors.
The incentive would be a belief that I could be part of influencing a descision for the better. My community has recently set up a Youth Forum to consider the needs of the young people in our area, with a plan to reduce youth crime and school droput rates. As a teacher and a parent I have joined this group as I may be able to facilitate change to better provide for our youth.
I'm going to agree with #7: if the group has no clear message -- and therefore no clear goal -- but is just gathering to gather, then even if I support them, what's the point? I see so many people fired up about "the issues," but few, if any, can articulate what they are fired up about. I considered joining a couple of political activist groups in the last three years, but ultimately I just couldn't budget the time -- which might mean that I didn't really care about their goals. On the other hand, if there was a group that advocated a goal I agreed with, and there was a very good chance that the group could legitimately effect change in that area... well, I might MAKE time for it. So far, that group hasn't appeared for me.
Effective presentation of a group's message is one factor that might (and did) help make me want to get involved with an interest group. A few years ago, I was in a large group watching a video put together by the local United Way agency. To say that the video was effective is an understatement; at the end of the presentation, many people were actually weeping. I got involved with the United Way partly as a result of watching that video and am very glad I did.
I agree with posts #2 and #3. I think the greatest incentive would be to join a group that I felt strongly about. If a group needs to persuade me by offering material incentives, I would think about what the group is really there for: simply numbers or to truly help.
Any incentive would have to be intrinsic, I would think. I suppose if I was working for the interest group as a lawyer, consultant or lobbyist, I might have an economic incentive to join the effort, but usually we find people in these (often single issue) groups because they realy believe in the cause the group is fighting for.
For me there would be few things that can motivate me. After all, I and the rest of the world is busy with life and other responsibilities. The best motivator though would be the cause of the interest group. If the cause is a great one, one that you can fully believe in and call others to do the same, I would probably join. Another incentive might be the fellowship of good people and company.
I think I would have to feel strongly about the issues an interest group was pursuing in order to join it. I don't think it would really matter if my joining was helping the group or not. If it was helping me, then it would be important. Just belonging, sharing, and feeling like I was a part of it is all the motivation I need.
A few years ago, I thought I had MS. So, I joined the MS Society for awhile. Even though I ended up not having the disease, it felt good to be numbered among them for awhile. And, I really learned a lot about MS. The MS Society might not be considered an interest group by some, but it seemed that way to me. I was interested in it, participated in the discussions, read the articles, and belonged for a time. It was good!
For me, the main incentive I would need would be my own personal attitude towards the issue at hand. In other words, if I believed strongly in the issue, I would participate even if there were not any material incentives for joining.
A second incentive that might move me to participate would be if people I knew and respected were part of that group. In that case, what would be motivating me would be a form of peer pressure. I would want them to think well of me and so I would (assuming I believed in the group's positions) participate.
Outside of that, I do not think there would be much that would motivate me. I do not really care to have power (so I wouldn't care about the possibility of being an important person in the movement). I can't imagine an interest group being able to give me enough in the way of tangible incentives to participate. Therefore, it would have to be emotional/social motives that would move me to participate.