For now we've always assumed that the group scaling mechanics work the same in Inferno as in normal, which is apparently incorrect. Information from the strategy guide has leaked to show us that monster damage and HP do increase per player the higher the difficulty. Even with these values, due to synergy a good group of friends is probably still better than solo (that is ignoring the henchman) but your random pick up group will be harder.
Normal Nightmare Hell Inferno Monster HP increase per additional player 75% 85% 95% 110% Monster XP increase per additional player 0% 0% 0% 0% Monster DMG increase per additional player 0% 5% 10% 15%
Here's an excellent post that sheds some light on the numbers: http://diablo.incgamers.com/blog/com...-it#more-19519