Wow, I am sorry you are having to deal with this. It must feel like a total invasion and make you feel completely out of control in your own home! I know how hard you must be biting your tongue.
I absolutely agree that adult children who move back home and expect (step)mom & dad to pick up the slack should be grateful, appreciative, respectful, and contribute. Making that happen is obviously more difficult.
Stephanie is totally right -- you and your husband have to be on the same sheet of music for this to work. I know what I'm talking about -- my stepdaughter, just last week, decided it would be easier to stay at home with us while going to community college than leave for the college she was planning to go to. She doesn't have a job, and wants to do a lot of "playing." My husband and I don't always see eye to eye on how to handle it, either. He is glad she is staying home and pretty much wants to roll out the red carpet. I am glad she's staying home, but feel like she needs to earn her own way, still have chores, and respect the house rules. She says, "I'm a grown up and can do what I want!" ahhhhhh!!!
You and your husband have to iron this out -- out of earshot of stepdaughter -- before you present her with conditions. The fact that she is 24 and still having these kinds of social issues does indicate a need for possible counseling, and I like Stephanie's idea of showing her how to be an adult. The drawback of being stepparents is that we often are not the only influence our kids have had in who they become.
If your husband is really resisting holding her accountable in any way, explore why he feels that way. Be non-threatening and non-accusatory. He may feel guilty or feel like somehow this is all his fault because he's not married to her mother. It could be that he didn't get to take care of her enough when she was little and this is fulfilling some need in him.
Make small concessions, but insist on some personal responsibility. Set goals that you and your husband can live with -- something like, she'll keep her job, she'll take a driver's course, she'll see a counselor, and she'll start doing this and this and this around the house. Re-evaluate where things are in a month and decide what else can be improved.
In the mean time, hang in there, and know you have a sympathetic shoulder to cry on!!