If it is accessible to the public and you have foot traffic they will sit there. My building has a similar situation and people make themselves comfortable all the time. My neighborhood is in a high crime area (Hempstead) Long Island. So sometimes there are unsavory characters hanging around. Also the grammar school is close by so each morning and afternoon ther is a parade of kids. The lawn is raised and they love to run up the stairs and walk past the garden. I enjoy the kids and I would never want to stop them from this simple pleasure. And I enjoy there presence- I am not thrilled about the drug people, but they don't sit there-they panhandle and leave. Then there is the general public and they sit because they are tired and it is a nice shady spot. I am OK with that also. As long as no damage is being done and it does not become a hangout I am fine with it. I don't know where you live or what type of foot traffic you have, but that should be a consideration. Also the investment of a few benchmark is not going to break the bank. If it attracts people you don't want, simple remove them. Or start small with one bench. I think that is a pretty simple solution to a simple problem. Hope you try the benches, it would be nice for the residents of your building. Oh yea if you want put a sign on them "private property".
Enjoy the fresh air