I have to add an AD group to a specific site collection, but need to give them permissions to be able to only view all list, libraries, video's, and to be able to download excel files, powerpoints, etc...?
You can add that group to a Site Collection Readers group in SharePoint permissions.
Site Actions -> Site Permissions
This will list the permissions for the site. You can add that group under the "Site Readers" and they can view and download the files they need to. Just click on that group and then go to
New -> Add Users -> type the AD Group -> Press OK.
Every user in that AD Group will be able to read the contents of that site.