Alright, so I'm trying to get the 'get every companion' quest, and I've got all of them, but left Cass until the end, as in a previous new vegas game she became impossible to remove (the option to part ways never came up, she just said something about not liking the 38 if I used the talk part of the wheel).
Anywhoo.
I've got all the other companions, but going to Cass' start location, she immediatly becomes hostile. I might be vilified against the NCR - but I've tried using the console to change that; nothing seemed to happen, and she kept attacking me (not to mention I'm neutral at the area she is in).
I've not been able to add her to my faction (as it says you can on the wiki), and the only thing I can do is stop her attacking (or revive her if I killed her - she was still hostile when she was revived again).

Is there anything I can do to get Cass to become a follower/companion type thing? I would do it on the save that I've already got Cass on, but it's impossible to remove her and I've still got a few more humans to make join on that one.
I would post the console commands I've tried, but I'm not sure if that counts as a spoiler or not.

Console... as in 360, or ps3? Or am I missing the point of that?

Anyways, if you're going for the achievement, and can't reload a previous save, make a new save, and get on the NCR's good side. Kill every C.L. you can find, just high enough to make the NCR not try to decimate you when you come into their surrounding area. Killing Powder Gangers works as well. That should help ^^

Edit:

There's also a quest in Freeside that requires you to recruit her. I'm not positive if that will give you a free pass to go try to pick her up, but it's worth a shot?

Console as in on the PC, the command console.

I figured it out though.
I changed the NPC rating to idiolized, killed cass, changed her to be friendly, revived her, saved and exited, turned it back on (as Steam wont allow you to collect achievements for sessions you've used the console on), and then talked to her.
Strange that she gets annoyed despite not being actvily affiliated with the NCR; none of the soldiers were attacking me.
Ah well. :3

Have you recieved the quest that tells you to go talk to Cassidy?