users registration across all sites


I have installed wordpress multisite - and will be creating sub sites for multiple cities:

I want to allow people to register for the website and have access to the main site plus all city sites. Is there a way to do this automatically?
I do not want users to be able to create a new site - only admin should be able to do that.

I am assuming I can then use the mailchimp integration plugin to merge the users on the website with the mailing list created through mailchimp. Is that correct?

Thanks for your help in advance!