My sims always end up owning a bunch of properties, but I've never really tried to interact with other sims at the non-home residential lots. I'm curious now, so I'll see if I can replicate your problem in my game, though I have several mods running and if I don't have a problem we'll have to get someone else to test it.
In the mean time, what expansions etc do you have?
Ok, I'm editing this to add that I went and invited someone to my house boat, and she was not able to come in. My sim was sitting in a chair and I tried the "invite over: chat" interaction. I tried forming a group with her, and she still couldn't come in. I don't have any help for you at the moment, but I'm at least confirming that the problem isn't just with you!
