Trying to recruit Cas but when i speak to her theres no option for it any help?

You have to give her a reason to leave the Outpost

Tie a stick with booze on it to her head and she'll follow that

Have you gotten the quest "Heartache by the Number" from her? Is so, it should say the following in the Quest tab on your Pip-Boy:

Cass is drowning her sorrows at the Mojave Outpost. Her advice? Go Work for the Crimson Caravan.

Go to the Crimson Caravan, and accept their job. Deliver an invoice to Hildern at Camp McCarran and return to McLafferty at the Crimson Caravan. She'll give you a job to buy Cassidy Caravans. Go to Cass, make her quit the caravan trade one way or another, and recruit her afterwards.

gotta work for crimson and convine her to sell her caravan.once you do that its easy with a few ways to convince including a drinking contest.word of warning i just done her quest for the first time and did it her way instead of the peaceful solution and it ruined my rep with ncr
so do it the ncr way if you wanna be mates with them