@montanapratt It sounds like you installed Sims 4 into your user's Applications folder, rather than the shared one. You should be able to drag and drop the app to the correct place. Open a Finder window and click on Macintosh HD, or whatever you've named your disk, in the left panel. (If you don't see it, click Finder in the upper left corner of your screen, then Preferences, then Sidebar; then under Devices, check off Hard Disks.)
In Macintosh HD, click Users, then the username that has TS4 installed, then Applications, and drag the Sims 4 icon to Applications in the left sidebar. Now the game should show as installed in all user accounts on your Mac.
As for sharing saves and other user-generated game data, all of it is stored in the Sims 4 folder inside Documents\Electronic Arts. Here's how to share files among users:
