Our current thought is to charge for business features. A Threadable list is a pretty open place by default. Lists are all members-only, but once you've been added you have a lot of control. Private (within the team) subgroups are a feature we'll likely charge for, along with Google Apps integration. We've also seen a lot of interest in a product that can be installed behind a firewall, but that's a little further off.
One thing we know for sure is that we won't charge based on the number of users in your organization. We really value people being able to use Threadable for large-scale organizing outside of the business context, and we don't want our pricing model to limit that.
