Open up the properties on the folder.
Select security tab | advanced | change permissions
check "include inheritable permissions" Check "replace all child object permissions..." so the contents of the folder will also be updated.
Select Add, select the group you wish to add if its not there already
Select the permissions you want (presumably "Full Control" which will check all the other boxes), ensure apply to has "This folder, subfolders and files" selected (and that "apply these permissions to objects and/or containers within this container only" is NOT checked).
Click OK to go back to the Advanced Security settings , click OK (which will change the permissions) and OK all the way out.
Go have a sandwich cause yer done.