@Rickie Rivera - Global Admin, Thanks for posting in Q&A.
From your description, I know you want to just assign one user to an app.
Based on my experience, there are only three options under Assignment session in Intune: Add group, Add all users, Add all devices, however, we can create a group that contains only one user, and then assign the app to that group, therefore one app can be assigned to a single user.
Hope it will help.
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