There was a post regarding part of this from Blizzard a day or two ago.
The maximum players in a group is 4.
In a 4 player group, if one person strays, they will die at the first big pack of mobs/rares/champions they encounter. While the 3 remaining might be able to carry on, but with factors like skill synergy, group makeup, dynamic and various other factors, they will likely die. So it pays to stay with the group.
Experience I would assume is calculated like most other games out there:
Mobs of your level give you a range of experience that is a function of your level and the enemy level with various other factors added in (factors like Rare, Champion etc. adding what I would suspect is a multiplier to the total because Rares, Champs give more XP.).
Then there will be a sliding scale as your level increases and the enemy decreases so you get less XP from lower level mobs.
Loot is individual and I am certain there is some math somewhere on Magic Find and standard drop %'s. Just search for any topic about Magic Find and you'll likely find some of the theory behind it there.