Both PokerStars and 888poker offer real money games so - yes, you can play online poker for money with your friends. However, this option might not be available if real money poker games online are not legal in your country or state.
The royal flush is a case of the straight flush. It can be formed 4 ways (one for each suit), giving it a probability of 0.000154% and odds of 649,739 : 1.