I was reading one of the faqs (absolute steves) and according to that quest line you can just pick her right up or so he puts it but no i didnt do them. So you have to do them first before she will join?
you don't HAVE to do the quest for ranger jackson first- you can beat her in a drinking contest or convince her to sell too.
that requires barter and speech challenges of 50 of either, or 75 of either though.
oh- and i lost with a 6 endurance, but won with an 8 endurance and the "i can stop anytime i want" perk for the drinking contest.
there is also a well-known glitch associated with the quests, DO NOT do the caravan quests and ranger jackson quest before speaking to cass for the first time or you will end up not bieng able to recruit her.
speak to cass before accepting the crimson caravan quest AND ranger jacksons quest- if you only accept one of them i believe it should be ok.