Different user acess for multisite sites

Hi. I’m totally new to Expression Engine. Actually so far just working on a build someone else did. Its a multisite install and it sounds like they are unable to give out full admin access to just one of the sites without exposing all of them. Currently we can’t change any templates, just basic content editing. Based on what I’ve read so far, it sounds like they could grant admin for one site without issue. I have not been able to locate a document on exactly how to set that up. Can you direct me to a help file or how to on setting different access for different multisite installs? Thanks so much.