Julia appears to be consumed with a desire to do wrong, to stand up to the party by sleeping around, by bedding members of the party, and then gets herself involved with "the brotherhood" because it also seems risky and wrong. She does not, however, like Winston have any sort of a political sense, the idea that somehow she could inspire real change by helping the proles rise up, etc. Hers is simply a desire, like the previous post says, to slap Big Brother.
But the release and the joy they find in breaking the laws together is something they share, not just the physical pleasure of their illicit affair but the fact that they are sharing in their crime, doing something together. It is this sense of togetherness that is lost in the end after their torture when they realize that they only care about themselves.