Users having one login for different sites

Hi, is it possible to make a user sign-up network wide? Currently I have a listings section where people can login and change their listings. I'm creating an events section too, though this is using a different theme, and I was wondering if it's possible for people to use their existing logins for this, and have it so that when they register they can use the same login for everything regardless of them being on different areas of the website?

Thanks!