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.