Hi Timothie,
If you don't want to share your web map and web app with all users in your organization but only a few people you don't have other option.
You can't assign users to each item in your content. It can only be managed by Groups.
One thing you can do is if you open your group, you have the option to choose the type of items you want to see (Maps, Layers, Apps, Tools, Files). This types can be used in the URL, just type &focus=maps after your group URL:
arcgis.com/home/group.html?owner=OWNER&title=GROUPTITLE&focus=maps
If users open this URL they will only see the web map types in the group.
Akos