Ok, so it's setup, but I am still able to access my homegroup folders via a non-homegroup device. Hell, I can get into folders that are set not to be shared. How the eff-? This is secure?
EDIT: Looks like my sharing settings are open. Turned those off, now testing again...
EDIT 2: Great. Now that my file sharing is turned off, the Homegroup was killed. So how the **** is Homegroup secure when my PCs are broadcasting in bright neon signs across the network to my room mate's PCs? I don't get it?
This is not how it works by default. You have some really goofy things going on.
Homegroup has three levels of sharing/security and three contexts of sharing. It is designed to be easy, so you are not exposing stuff to people that do not have permission.
First clean up your security:
Kill the Homegroup on all the computers. (Leave homegroup)
If there are any locations that are still appearing on other machines - go to the system where the folder exists, right click on it, and select: Share with - Stop Sharing (Share with - Nobody on Windows 7)
Do this until there is nothing being shared or visible from other machines.
Next recreate a new Homegroup and only share the libraries you want other PEOPLE to be able to see.
If you want to 'share' additional things for ONLY yourself to see, then right click on that folder or library, and Select - Share with - Specific People - Add your login name, and set what level of permissions you want to have.
Using the 'Share with' option when right clicking you can easily 'Add' or 'Remove' any location for other HomeGroup or specific users to see, and what level of access they get.
This is what makes Homegroup simple, as you can publically share things, like a family would in a 'Home' and all anyone has to do is join the Homegroup to gain access.
Homegroup also facilitates the sharing of Printers and Devices as well. So you can setup a Homegroup and literally only Share Printers, giving everyone access to your printers, but nothing else.
Homegroup is also where you can control your media sharing options. For example DLNA type sharing to Media devices, Xbox, PS3, etc. You can allow or block each device on your network specifically.
With Windows security systems, setting up shared folders can be a bit much or a non-tech person because it includes several layers of security, even when using the 'simple' GUI contexts of File sharing permissions and File/Folder permissions.. (Basically it is taking care of: Kernel tokens, user account, ACLs, NTFS File/Folder permissions, network sharing permissions and firewall exceptions.)
The Sharing Wizard (Share with) and Homegroup makes this far, far easier and ensure the security is right as well.
I hope this helps out a bit. The best thing is you can always dump Homegroup (As I suggest above, and start again, or adjust specific files and folders using the Share with option. (Even if you have shared your entire picture library with your friends, if there is a folder inside it that you do not want other people to see, right click on it and select: Share With - Stop Sharing.)