I claimed my 18 year old daughter as a dependent (she's still in high school), she also works and has a daughter of her own can she claim her as a dependent?

Sorry, I don't think so; aslongas you claimed your daughter as your dependent on your return, your daughter , as your dependent,can't claim her child as her dependent on her return.However, even if she is your dependent, when she meets the requirements, then she still needs to/can file her own return as a dependent.