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)