IC participants should NOT be automatically added to the Group Buy. An IC is just to guage interest, there are no actual commitments made at the IC stage, although it's requested that anyone who signs up for an IC should be willing to commit to the GB, it's not mandatory and there are often changes from IC to GB that may change the person's interest (price changes, design changes, etc), or sometimes changes to the person's situation (job loss, unexpected expenses, etc) or simply the timing of the GB (other GB's the person has signed up for recently, timing of the GB, salary payment dates, etc).
What tofgerl is proposing is a very good idea, but it means there would be three stages to a GB:
1. Desiging the set / keycaps / PCB's, etc with community feedback (this often happens currently in IC threads)
2. Actual commitments to purchase as an "IC" form.
3. GB stage.
The last 2 should really be part of the GB, not the IC, IMHO. Something like Massdrop's "commit at this price point" is a more flexible idea to me, and covers all cases. If the numbers don't reach the goal, the GB doesn't happen. If it does, everyone gets invoiced at the final price (at least all those who commited to that price point or earlier ones). People who commited at an unreached price point will not be included or invoiced.
The way I see it, an IC (the way they currently work) is more for feedback for the designer at the stage it's still in the design process than for actually running the GB on a final product / design. Of course, I could well be wrong on this, but then we may need a new subforum for "Design Feedback / Active Designs / GB WIP" or something like that. Sometimes it makes sense to get numbers on the interested parties, sometimes not.
So, in my view the IC stage is more of a Work-In-Progress / Feedback / Interest Check, with the numbers slowly growing, but the actual interested parties changing over time as the design progresses (as a design change would turn some people off who prefered the original design, but usually would be made to accommodate a larger group of people). So if you do implement an IC section, perhaps it could be something a user can opt out of if they change their mind? Of course I think designers prefer this to happen in a thread where the person can voice the reason they've lost interest.
Some designers may find value in an IC section to count interest on a particular "finalised" design, though, so it could be a handy feature, but probably won't be used as much as the GB section.