You can restrict the applications that a user has access to - This article provides information on the "assigned users" feature.
The "assigning users" feature is performed by Administrators and Power User and is accessed through ASSIGNED USERS on the left-hand menu for each application.
For the bulk assignment of users to multiple applications refer to the KB article - How do I assign users to multiple applications?
- On the Applications page choose the application that you want to assign users to
- Click ASSIGNED USERS on the left-hand menu, then ASSIGN USER
- Place a tick next to the name of the user or users that you want to assign
- Click ASSIGN SELECTED
You have successfully assigned users to a single application.