How to give an ftp user write permissions for a select site (DreamHost if that makes a difference)?

So if the filepath when logged in via SFTP is home/ftpuser/sitename/ how do you set it so ftpuser has write permissions for all things in sitename? And ideally in sitename2 as well, but not sitename3?

I see the permissions for directories are set to 755, and the files are 644, but I don’t understand what to do next.

Something to do with SSH and adding the user to a group or something? Creating a group first? I’ve tried googling and just get confused.

I just want to set it up so ftpuser can go in SFTP and edit functions.php or upload a php.ini, or whatever they need to. As of right now they can download and view things, but not edit.

Note I am ftpuser and am requesting this info for the actual owner, so I need an explanation that makes sense without being able to try it myself, and simply being the owner account doesn’t work (unless you can have multiple owners?).