When you first create a Group and give it access to the Resource Library module, none of the existing Documents in the Library will be accessible. This is because the new Group hasn't been given permissions to any Documents. While you could now edit each Document and select the new Group in the permissions area, there's a much easier way to assign permissions using the Library Report.
Step 1: Go to Library > Reports
Step 2: Go To The Library Report
Step 3: Select Documents To Change Permissions
Step 4: Select Groups > Save
What if I have more than one Library?
If you have more than one Library, you'll repeat the above process for each Library.