I can't get the girl(Cassidy) to follow, or even got to Freeside for the "Birds of a Feather" quest, please someone help here...

Anyone?

I am currently working on this too!! but acording to the guide, you have to go to camp macarren and talk to the doctor there and then go back to the caravan and talk to the old lady....then make sure that you have an open spot for companions (for she will be one) and take her back....
I am righ now getting ready to do this...i will post again if it works

Posted from earlier source....

You'll need to do the first quest for Alice McLafferty/Crimson Caravan's, which is delivering a holotape or something to Doctor Hildern at Camp McCarren. After you do that, you should be able to get another quest from Alice, about convincing Cass to sell her company. Then you'll just need to convince Cass to sell, and ask her to come with you.