Hope I'm not speaking out of turn here, but here are the basics [as far as I know]:
BlockParty is the name of a series of group [hashrate] rentals, where participants send the person organizing these rentals [Willi] their BTC.

If a Block is found the Block Reward is shared proportionately based on participant's entry level [ie if you sent .01 BTC and the total BlockParty amount raised/used in the rental is .15 BTC, you would get 1/15 of the total BTC Reward of 3.xx BTC].

Over the past several [6-8?] years, BlockParty rentals have hit [I believe] 10 Blocks and have a history of paying out to all participants, at least from what I've read and seen here (over the past 18 months of following/monitoring this forum).

Hope this helps