Realms mode supports online play for up to 10 simultaneous players within a single world, allowing friends to collaborate on projects and/or raise havoc together. The alpha version of Realms is currently free, and will be an optional paid service in future updates.
The latest Pocket Edition update also adds features such as buckets, eggs, milk, cake and fire to the game, making it important to keep an eye out for pyromaniacs among your online guests. Maybe they'll play nice if you bake them all cakes beforehand? Just an idea.