How to share a users between projects?

[RedTTG]

I made an organization for my website. I'd like to renovate the site and add my own all-in-one accounts as they are called to allow users to stay authenticated across different services on my site, how can this be setup? And additionally what are some website authentication setup leads anyone can give me since I don't specialize in websites too much but I want to give my site a upgrade for that reason