If for example you want the youth minister to only be able to login and edit the youth ministry page, you must do the following.
Login to the backend control at www.yourdomainname.com/administrator (where yourdomainname.com is your actual domain name).
Go to site>user manager.
Click on the persons name you want to give editing priviledges to.
You will see a field there called 'group.' You want to make this person an AUTHOR.
Now go to the page you want them to be able to edit by going to content>all content items. Click on the page you want to assign.
On the right hand side of the screen you will see the paramaters section. You will see a field called 'change creator' and choose the staff members name you want to edit that page from the dropdown menu.