Hi Malin! Glad you're enjoying the plugin; thank you for the KUDOS!
When you enter multiple list-ID's in one of the membership levels, the user will be subscribed to all of those lists when they join. To my knowledge, it's not possible to subscribe people to different lists based on their Custom Capability.
Regarding Constant Contact: s2Member doesn't currently support that list service. Only MailChimp and AWeber are supported. However, if you really need that integration, it's possible to write some custom code to do the integration:
Other List Server Integrations ( there's always a way )
Check the s2Member API Notifications panel. You'll find additional layers of automation available through the use of the `Signup`, `Registration`, `Payment`, `EOT/Deletion`, `Refund/Reversal`, and `Specific Post/Page` Notifications that are available to you through the s2Member API. These make it possible to integrate with 3rd party applications; like list servers, affiliate programs, and other back-office routines; in more advanced ways. You will probably need to get help from a web developer though. s2Member API Notifications require some light PHP scripting by someone familiar with web service connections.