The buff affects all party members within 200 yards of you. There seem to be no notifications of who is providing the buff to the group, plus there are no warnings when players with XP on invite others with XP off.
This affects the very popular experience gain method of
자유지대 boosting, where a level 110 Demon Hunter with the right gear and legendaries is able to boost someone from level 110 to 120 in just 2 hours.
Buuloki previously explained how leveling with level 110 twinks works in a comment:
The most common thing to do was have a 110 twink for island carries. You'd use legendaries, your artifact, gear it up as much as you can (mythic ABT carries usually) and gem with BFA gems (up until that was hotfixed).
Then, most classes could pretty much solo islands. Recently been running with a 110 twink Warlock, he just slows the entire island, casts dots and runs around himself. The mobs never reach him and just die off. A whole normal is done in maybe 5 minutes? Brewmaster is also really good too, triples my damage at 120.
This is very much a "fun detected" move imo, I really did enjoy running with twinks, islands were still interactive, i still helped out, and gearing a twink would of been fun if this was not implemented.
Perhaps a better fix would of been to readjust scaling at 110 for islands, rather than disable it entirely.