This is not how it works anymore. Sometime recently (on purpose or by accident?) the devs changed the sharing system in a couple of ways. Diablo 3 Party Experience Stealth Buffed:
1) There’s a “stickiness” now so that when characters are all together in the same area for some time, they will continue to share exp (for a short duration?) if they move apart. There are conflicting player anecdotes on just how long the stickiness lasts, but many people have noticed they continue to gain exp while in town if everyone else remains in the dungeon they just left. The stickiness seems to be Act-wide (so you’re still sticky in Tristram if others are in Fields of Misery), but not from Rifts back to town. More testing on this is required.
2) Whether sticky or not, EXP shares throughout entire zones, not just within ~100 yards, as previously. (Zone = same level, e.g. Fields of Misery, or Halls of Agony lvl 2, or a single level of a rift, etc) That’s why that chart at the top isn’t true anymore, and this has prompted some changes in player activity. For instance, party clears of large areas, like the Fields of Misery, are now a very good (arguably the best) way to earn big EXP. (Though it requires coordination on joining and leaving the game and patience to to D2-style same level grinding over and over again.)
Previously you had to be close together to share exp in big zones, but now you can all go different directions and still gain exp for what other players kill, and doing that in large areas with a ton of enemies, so all 3 or 4 people in the game are killing non-stop, racks up giant exp gains. This can be combined with key farming or bounty farming, if players coordinate to spread out through the same large areas at the same time.
3) Cursed Chests and other event exp gains are global, so everyone in the game gets them no matter what act or area they’re in. This was always the case if you were in the same act, but seems to be newly expanded to anywhere so long as you’re in the same game.
All these observations are fairly new and anecdotal, and there are some conflicting players reports, so more experimentation and data-gathering will help us figure out the undocumented change.
Click through for video evidence.
Thanks to EmperianGaming for uploading a video of this technique. The vid’s gameplay is boring, but it does document the shared massive exp. Note that doing this instead of Rifts will net you far fewer legendary items, so it’s really just an exp farming technique.