Putting away special toys and practicing how to share before friends come over really helps prevent problems during a playgroup.
Before friends come over we practice what to say while sharing. Some of the phrases we teach are:
“Let’s take turns!”
"I'm using it right now. I'll let you know when I'm done."
"We can play with it together!"
"Ok, you can have a turn first and I'll have the next turn."
That way when something is fought over, those phrases we reviewed are still fresh on her mind. Although it is a simple concept, practicing how to talk about sharing is really effective preparation for time with friends.
Based on an idea from: