Mailchimp Integration + PopUp - cannot use multiple lists or groups?

It seems as though it's impossible to use multiple lists or groups with Mailchimp Sync. I'm using it in tandem with PopUp Pro, and while you can select any number of groups, it seems as though you can't specify which group a given subsite's signups should land in. I have 6 sites and instead of all 6 routing to the main list, I need each site's signups to go to that specific site's list.

Am I missing something or is this not possible? Thank you,