Hey Christian,
the way I mentioned is classed as 'Single optin'.
The member only has to click the link in order to opt in, thus do just ONE thing.
If you set up 'Double Opt-in' in S2Member then that puts a tick box on the registration form that a member has to tick at sign up just in order to be sent the email opt in link. The user ticks the box at sign up and STILL HAS TO CLICK THE LINK in order to be signed up, thus a double opt-in.
I don't know of any way except for manually inputting members emails within Mailchimp to sign them up without their express permission.
Ross
