I haven’t seen a big discussion on the forums about group bonus XP. I think if Blizzard where to offer a scaled increase in XP based on the number of players in group for grouping in competitive or matchmaking in general it would greatly incintivise playing in groups.
A basic model could look something like this:
2 players - 20%
3 players - 25%
4 players - 30%
5 players - 35%
6 players - 40%
Seriously though, people will do anything for a slightly faster way to gain loot boxes simply from playing the game in groups rather than in a solo que environment.