I'm still new to CraftCMS and still learning about site structure. The site that I am asking about below is not (yet?) in CraftCMS: Our site is composed of many subsites which have different users contributing to them. Would it be possible within CraftCMS to restrict a user's login to only viewing the content that they can edit (ie- within their 'subsite' area)? I say viewing (not editing) since we have a LOT of content and I'd like to keep the content accessibility separate for different users to prevent confusion.
You can certainly restrict user's access to specific sections on a Craft site, using Craft Pro's user and user group permissions, both in the control panel and on the front-end.
Lots of different ways to setup these "sub-sites", but it sounds like you'll be wanting them to share the same Craft installation.