According to the latest patch notes we can now convince the Followers to aid the NCR at Hoover Dam and get the ending where they are allowed to stay. Has anyone figured out how to make this happen? All I know so far is Julie doesn't immediately have any conversation options about it.

Get idolized with both of them and talk to Julie.

I'm guessing you have to be at a certain point in the NCR questline and you need a certain rep with the Followers. I had an old save I opened up where I only had negative rep with the Followers thanks to The White Wash and she denied the offer. Not sure what rep level is needed exactly.

The main quest is irrelevant. I convinced her before I got to the strip. It's all about reputation.