I spent quite some time trying multiple group combos and recorded the data :
https://docs.google.com/spreadsheets...it?usp=sharing
takeaways / surprises (for me):
You can be in raid and kill out in the world in Classic and get xp as long as everyone hits the mob
Group bonus for 5 and 4 are the same (or very close/ in noise)
Whichever number of character, the total xp extracted from a single mob is always higher with more characters in party/raid
In BfA you actually get MORE xp in per toon in a raid than 2x4 (at the cost of having to land dmg on all 8 to get that xp)
And reminder: you can enter most dungeons in raid of 10 in Classic and then don't need to have all hit a mob to share the xp (waiting for someone to confirm there is also no change to the above multipliers in that case)