Greetings,

Regarding XP Group Bonus, this thread on WowWiki.
http://www.wowwiki.com/Formulas:Mob_XP

Group Bonus :

[code:1]1 person group = 1.
2 person group = 1.
3 person group = 1.166.
4 person group = 1.3.
5 person group = 1.4.
[/code:1]

There's also a part about XP gained when powerleveled by a lvl 70 but it's un-grouped.

/salute