Sharing is something that is worked on a lot in kindergarten. Most children have a firm understanding of the concept by that time, but it doesn't mean they necessarily like to do it. When I taught I had community supplies on all of the tables as well as many other things that encouraged them to share items in the classroom. I usually discouraged them from bringing their "at-home toys" to school so that wasn't an issue either.
I've started working on sharing with Blake and have since he began playing. We do your turn / my turn with the toys. He is pretty good about it, and there are a lot of times when he now will bring toys to me or others over to visit to let them have a turn. It's a little tougher when there are other children around as he has limited exposure to other children his age. But, he is starting to understand how that works too.
The challenge is when I have something and won't share it with him... yeah that doesn't fly very well. For example, Blake wants to use my camera. He loveessss to look at the pictures after I've taken them and has for a while, but now he wants to use the camera too. We tried giving him another camera (not a toy camera but a real one that no longer works) yeah that's not what he wants. So now it's hard because when I take out the camera I can get 1 or 2 pics and then he either wants to look at the pictures or take his own. Hmmm I think a camera for Blake will be in order for his birthday, if he and I can last that long!
Blake and his camera:
The picture Blake took with my camera (with a bit of help):