Count me among those who don't post often but always shop at the refer a friend sign-ups. In fact I have been participating with Cyanne's groups since before she started doing them on the smartcanucks forum.
Remember to keep things in perspective. If for some unfathomable reason you were the only person to shop in your whole group you would still get 2500 points since those 5000 points are only being split between you and your group leader. The difference between how many points you get for 8 or 9 people shopping vs. the full 10 is minimal.
Keep in mind that none of your groups points will be distributed to the others who did not shop. There is no gain for anyone who would sign-up without any intentions of shopping, therefore participation is very high in these events. Usually there are only a couple of people who sign up and don't shop, even then I think they had every intention of shopping and then somehow life just got in the way.
Hope this makes those who are nervous feel a little more at ease.