Yes, that's true that leaving and joing back will get you more exp, but the part you don't get any exp with full 8 ppl in hell game part during second wave is bullshit.
Actually you do get exp in 2nd wave even if partied with all 8. You get 1 exp with each skele kill and about 10k+ not sure guessing each raveler kill. But if with 7 peeps its diff.
any memeber can leave, it's just that if you are lvl 90, the first two wave you don't get any exp at all, and if you are a lvl 70, you still get some good exp.
this is a myth, its just some stupid trick to get the highest level ppl to leave cuz they think they take all the EXP, in actuallity EXP is shared and ppl in the game ONLY affects the * to EXP modifier. its funny what noobs come up with. All you proof is in diabloii.net, ive read it ages ago.