Fallout new vegas recruiting cass?

I'm doing the 'birds of a feather' quest and I'm at the bit where you go to the mojave outpost and you have to talk to cass. However she appears as an enemy on the radar and she shoots me when I enter the barracks, because the NCR don't like me. Even when I'm wearing NCR patrol armour she shoots me because she's a ranger. Is there any way around this or any other way to recruit cass? I've just got the other 7 companions and now I can't get the achievement.