Multicity selection

Hi,

This is my first post in the forum, I am dealing with a complicated situation so i thought why not to ask some pros.

So, here it goes:

I have set up a multisite, i am using two themes in total 1 for the mainsite.com and other for various cities (sites) multisite.com/london, multisite.com/newyork and so on.

What I want is that the user should sign up on mainsite.com just like they offer on groupon.com which is, they ask for email and then city. so they can send emails to the user according to the city selected….and this info is stored in cookies so when the next time the user opens groupon.com it takes them directly to their selected city.

I wanna achieve something like this too.
Can anyone please tell me how to achieve the same, with a plugin or code or ext. library. I know very basic of php & javascript…good handle of html and css. Any help would be appreciated.