Why in the world does Julia agree to go with Winston? Doesn't she know the dangers?
Julia is well aware of her situation. She agrees to rebel against the Party for different reasons than Winston does. She does it because she feels she has a right to live the way she wants to, and her "flings" (she's had many) are part of that fun. Winston rebels because he feels the way they live is insufferable and that active rebellion and overthrow of the Party is the only way to change it. She thinks this is silly and that of course the Party will always be in control, but that one may do small things like finding secure hiding places within which to be intimate with a like-minded person in order to rebel privately.
Does she know the dangers? Surely she does. However, it is not quite certain in the book if she knew the extent of the situation until the hidden telescreen in their rented apartment is disclosed and they are captured, separated, and taken to Room 101 where the deepest fears of a person are forced upon him or her in order to coerce him or her into obeying the Party fully once again.