Need Help with Raspbian SFTP Permission (Noob)

[Calico Morgan]

Hello,

I'm doing a bit of work on an SFTP server as my way of noodling around with Linux for the first time! I'm having fun with it so far, but I ran into a bit of trouble. I have a handful of users and groups, and I'm trying to get them exclusive permissions to their namesake directories on a USB hard drive. Directories are all set on the hard drive, groups are arranged how I want them. I'm just wondering how I address specific groups when I give them permissions for directories. So far I found: 

sudo chmod g+rwx /media/pi/PiCloud/guest

I understand that this gives the group read, write, and execute permissions to that directory, which is what I want, but how do I specify which group I'm granting that to? I want to give group "guest" the above permissions. 

In case it's important, I'll list all users and groups bellow:
User: pi   --belongs to groups: pi, morgan, jordan, ryan, administration, eryn, guest
User: jordan  --belongs to groups: administration, guest, jordan
User: eryn  --belongs to groups: eryn, guest
User: morgan  --belongs to groups: morgan, guest
User: ryan  --belongs to groups: ryan, guest
User: guest  --belongs to group: guest

I have folders in my hard drive called:
Administration
Eryn
Guest
Jordan

Morgan
Ryan

So I'm trying to get each user/group permission to rwx their name-sake directories, but I as pi need to be able to have access too. Is it possible to do this with the above example, or am I way off base?

If I haven't triggered you too hard with my sheer ignorance, I'd really appreciate some pointers!

Thanks, and let me know if you need more clarification. 

[Chunchunmaru_]

Every file/folder on UNIX has an user:group permission set, but can also have a single user or a single group
For example

-rw-r--r-- 1 foo:bar FOLDER1 tells us that FOLDER1 belongs to foo(user):bar(group)
let's say in /etc/group you got this
 

bar:x:1:user1,user2,user3

those 3 users belonging to the bar group can has the overall group permission

depending on the SECOND permission set you got on that file, you have only the r as you can see in the second file permission, -rw-r--r-- means the group you specified can only read access

So, TL;DR
that file/folder you want to give access to the guest group you need to "sudo chown *user*:guest *filename*" and then you need to set the second permission set with chmod