Step I: Go to the Azure Admin portal at https://portal.azure.com
Step II: Select the Azure AD tab and then select the Decisions application under enterprise applications
Step III: Go to properties and set the “User assignment required” to “Yes” and hit “Save”
Step IV: Select “Users and groups”
Step V: Add the users who should have access to the Decisions Application
Note: Only users (or groups) listed here will have access to use Decisions.
Learn more from Microsoft: Assign users and groups to an application in Azure Active Directory