In my experience, you have to sit there with them and demonstrate sharing. This takes a good little chunk of time though.
We'll play a game where everybody has a toy, and there is a timer for 5 minutes, (or even better, a song, like musical chairs - musical toys) when the timer goes off, or the song changes - everybody swaps toys!
Now, on the other hand my daughter is 8, and my son is 2. There are toys that are totally inappropriate for the 8 yr old to be playing with (I don't know if she is regressing or intentionally trying to annoy the 2 yr old) I make her give those back. And if Seth takes one of Jordan's "girlie" toys or special toys - he has to give those back as well. I do believe there is a thing as "special" property. But those things should be put away before guest come over becuase it is not fair to have toys out that EVERYONE can't play with.