I think you need to sit down and explain expectations. With my SS, we said if he's going to be home and it is not a week/day we expect him, we need to know before DH goes looking for an intruder or I call the cops. One time SS was home and I found out at 3AM. We talked to him later about why that wasn't a great thing and how that really scared me to hear a noise downstairs when we didn't think he was here. We wanted him to be home vs driving back to campus tired, but he needed to call first. Oh, and one night I was sick and out on the couch at 11:30 PM and he brought friends over. I was really angry that I couldn't just be sick on my own couch without him bringing in an audience. We had to discuss that with him, too. I admit I wasn't too friendly that time. I didn't talk to him in front of them, but I did talk to him later about how I felt. In our case, we had a teenager in the home til this fall and we still have a young child. It is our home, not a pit stop.
If they are using things that you do not want them to use, tell them it's off limits. Tell them that they need to ASK.
My DH tells of a time after he moved out where he couldn't key in (his parents had used a deadbolt he didn't have a key for.) He looked through the mail slot to see his dad's nekkid butt running for another room. He learned a lot about respecting their privacy after that.
I'd sit them down and say, "This will always be a home for you, but we need to redefine what that means because you no longer live here full time. I'm speaking to you both as your mother and as an adult. I need you to (call ahead, ask to bring friends over, not come and go as you please, ask to use my things). You would not want me to wander in and out of your home, I'm sure. This is not a group home or dorm. We do not keep weird hours. We are not a grocery store or quickie mart. This is a family home and I need more respect from you or I need your key. It's not that I don't love you, but you are not teenagers anymore and things need to be changed." You might include things like "if you come in and eat our food often, without asking, you will be asked to contribute/replace the food." It is no fun to go into the fridge and find that DH's lunch is gone because SS came by and got hungry. Our grocery bill is very different when SS is here. We also said that it wasn't that we wouldn't support him, but he's an adult and if he is here often, then all our bills go up and he should contribute, the same as DH and I do. If he lived here past this summer, he was going to be paying rent (not much but enough to pay DD's tuition). He still has a room here, but post-grad we made it clear that things were a changin'.
I would not dream of just showing up in my mom's house and taking anything I wanted and bringing friends over to hang out and eat her food. This is a time of change and a time to redefine the parent/child relationship. It may be bumpy, but I hope you all can come to a better arrangement about this. It may not seem very nice from their POV, but they aren't little kids anymore. Why should they act like it?
If you think that some of them would take the info better from a bioparent, then do it that way and speak as a unit (this is what WE want for OUR home).
Edit to add: I wonder how much is cultural. If you regularly accept drop ins and think nothing of adding a plate to the dinner table, are you more likely to not see the issue this OP faces? In my life in general, I want a call. I do not drop in without a call. My ILs are frail and sometimes they are too sick to see DD even though they love her. Even though they are welcoming. We ask SS for what we give to others ourselves. So maybe it's not heartless or unloving but just wanting to have a little notice so you aren't walking around in a towel (or nothing) in front of strangers that the kids walked in with. SS could call up and say he'll be here in 10, have we eaten yet? and we'd make room for him. But I want the call.