office 365 groups and sharepoint

[sted]

im trying to create a sharepoint site to use as a shared document store (read probably end up a dumping ground) i can create the site but i cant seem to assign groups to it(just individual users). office 365 set up to sync from ad so i have various staff groups that have synced across to office365 but when i try adding any of them to the site they dont appear as a selectable option all i have is users or office 365 native groups with an email address. Is there any way to get groups to sync in a way i can use them for 365 permissions as adding users individually seems dumb and there has to be a better way even if i somehow add my ad group to a 365 group but i cant see to do that either.