Connecting Site offices with Main Office

Hi,
i want to know complete detail of connecting site office with main office.

i have 50 user in main office with DC & all user loging to the main office domain & working fine.
now we are setting up a site office & want the users ( aprx. 25 users) to login to our main Domain.

So what are all the possible ways & steps to configure the site office so it communicate with main office & i can manage the user at both end from any office without any problem?

i want specificaly the deployment method at the site office to main office. i have good knowledge in setting up separately at each site.

thanks,

Faisal.