Yes the group mines without a pool but with their combined rented hashpower pointed towards -ck solo mining pool with the supplied Group mining address/es to check the stats.
The idea is to maximise ones chance of solving a full 25BTC solo block but dividing the winnngs between the group and how much each user invested.
And that is only if the group finds a block, you may or may not find a block per round but statistics show that it is more probable/likely.
Essentially all your rented hash power can be directed to a solo pool creating a single or multiple workers (like a normal pool) but you are mining by yourself so all shares made by the address/miner are trying to find a share with over the bitcoin difficulty thus finding a block.
It's a basic lotto syndicate but with solo mining for bitcoin for the jackpot.