The easiest way to share files with only selected individuals in a group is to create a project within that group. Choose the "Projects" link in the left-side navigation bar of your group.
At the top of the list of projects associated with your group is an "Add Project" button that will let you create a new project.
In the new project, click the option to add only selected group members, then choose the members that you would like to add.
Use the “Files” function to share the files.
Save the project. The project members will be notified that a new project is available to them.