The very first 'dream sharing' experience I had, someone else entered my dream. I was not lucid at the time. I was just going about my business in the dream when suddenly...holy shit...there was a PERSON standing right in front of me. I mean, a REAL PERSON. It seriously startled me and felt very invasive (even though I had given this person permission to enter my dream and we had been attempting to dream share for awhile). It was so incredibly strange. Like imagine that you are alone in your shower, washing your hair, singing like an idiot...when a person just materializes right in front of you. That's how it felt. The first time.

The conversation I had with the person the next day was strange. He asked me if I remembered the dream and I said yes, you scared me. He was offended that his appearance would scare me, but eventually understood when I was able to explain it.

After that most of my dream sharing instances were much less "interactive" ...where I was actually more like a ghost just floating through their dream. I would try to contact them or interact with them, but they couldn't see me at all. I could only make observations about the surroundings and attempt to remember as much as I could so I could ask them about the dream the next day and see if I was actually there or not. Though, I have managed to wake my sister from her sleep by screaming at her even though she couldn't see me.

So my conclusion is that...dream sharing is weird, and can be experienced differently each time. Expect anything.