I have a friend that told me that she went to her Personal space, as soon as she gets there, she sees someone standing over by some furniture in his navigator, his avi was apparently fully loaded as well. She said she tried to invite a friend and she was immediately booted ! If she was hacked, which I doubt, could they use your spaces with you logged on simultaneously? Is she lying to us?
I had something similar happen months ago and I remember some other users reporting it also. Some sort of merge occurred and there was another user in my dolphy room. We spoke briefly and both asked each other what space we were in (he was in a different personal space altogether). He could walk through walls and barriers and apparently I did the same on his screen. I didn't try to invite anyone so I have no idea what the result would have been that way. He poofed after a couple mins and I never saw the glitch happen again. I don't think a "hack" exists to have this occur...most likely it was a rare glitch that your friend will never see again.
Here's a couple other threads about the same thing: