multi-site conection (Sigle login for multiple sites)

So... I'm going to be working on a website that will consist of multiple sites conected as one large website. (ex: domain.com,item1.domain.com, item2.domain.com) but I want to set it up where it onlyrequires to sign into any of the sites (connected accounts) and not have to sign in again when switching from(let's say) domain.com to item1.domain.com I know wordpress multi site is an option, but I honestly like to say away from wordpress if possible and I know it can be done. I'm guessing it'sdatabase related ssuebut not sure on how to go at it as I've never done this kind of connection before and figured I'd see if someone on here has the answers I need.