How to do group buys on GOG "Pick X, Pay Y" bundles:
There are two ways to do group buys on GOG, a "harder, less restricted" way and an "easier, more restricted, always works" way. The "harder, less restricted, YMMV" method requires intervention from GOG support, which is why I say it's "harder". Basically, in order to do a group buy regularly, each person in the group buy must own all of the games each subsequent person wants.
For instance, if you have a host (X), a secondary buyer who will be redeeming the code second (Y), and a third buyer who will be redeeming the code third (Z), then X will need to own the games that both Y and Z want, and Y will need to own the games that Z wants.
The reason you'll be contacting support is to separate the gift code into N amount of codes (all of the stories I've heard have been about splitting the code into 2 codes), each with certain games, which you specify, contained in them. It's worked for splitting codes into 2, so I'm assuming that they can do multiple codes (3, 4, ...) as well.
It is better to do this with people that you can trust, because it relies on trust to distribute the code(s) correctly. GOG group buys rely heavily on code distribution, so if one person screws up, the whole group buy can end up being a disaster in the end.
Step-by-step instructions for each method
Harder, less restriction, YMMV
1) Get together a group of people who are interested in any combination of different games from the bundle.
2) Anyone can host. Host buys the bundle with all of the games that everyone wants in it as a gift and sends it to his own email address.
3) The host does not redeem the code, and contacts support asking them to split the code into N amount of codes, each with the selection of games from the bundle that each person in the group buy wants.
4) The group waits out the support request, and assuming support is able to fulfill the group's request, the host then distributes the keys according to which has which participants games in them.
Easier, less restriction, always works
1) Get together a group of people who are interested in a combination of games such that each participant owns all of the games that each subsequent participant is interested in. For the host, this means everyone's games, and for the last person this means he/she doesn't need to own anything.
2) Decide who will host according to the rules outlined above, and the host will buy the bundle as a gift, and sends the gift to his/her own email address.
3) The host will grab the code and redeem it for the games that he/she wants.
4) The host and each person thereafter will distribute the code to the next person in line to redeem the code until the last person redeems the code. Make sure that the order is clear and concise, because the success of the group buy depends on this. Also, keep in mind that all GOG codes have a 10 minute delay for redemptions after each time the code is redeemed.
mind games. 
