I asked a similar question on the Mass Email Sender forum, but it pertains to Mailchimp too.
Can we have a select box to chose type of users (user level) that will sync with Mailchimp?
By default, ANY user registered on ANY site gets synced to Mailchimp. However, many of the Mailchimp campaigns I (the network admin) need to send pertain to the site owner/admin, and not necessarily their authors or their subscribers.
In fact, their authors or their subscribers don't even know we exist behind the scenes in many cases.
For example, several of our (sub-blog) users have MarketPress stores, and when someone registers on their site as a part of the checkout process, they then become eligible to get synced with our Mailchimp list. 99% of the time those users (along with most subscribers, contributors, and authors) don't need to be on our Mailchimp list.