User Login

Hi,

I want to create a multisite:
https://www.mysite.com
– https://www.mysite.com/subsite1
– https://www.mysite.com/subsite2

1.how can i avoid that users that already are registered on the main site, have to register again on at the subsite, i want that the can login everywhere with 1 account
2. is there a problem if i use different themes for each site?

Thanks,
Erik.