Yellow Yak is a simple app that does just what you want.
1.) You can easily calculate a restaurant bill with a tip.
2.) You can split the bill with up to 25 people.
What more could you possibly need from a tip calculator and bill splitter? Wait. Ok, so you can do one more thing. You can hit "Random Tip" and it will add a tip between 10% - 30%.
This feature is more for fun, and if you use it, who knows, you might be inclined to give a 29% tip to your server!