Yeah I guess you could pay via PSN cards but that might make things difficult. The game groups are where you split the price of a game (or multiple games) between 5 people. Whoever happens to be buying the game (host), will be receiving the payments from the other group members (via paypal), so he can get the game.
Ex:
inFAMOUS 2 group($59.99)- $12 per person
1. harleymandvdson (host)
2. someone
3. someone
4. someone
5. someone
All members (excluding the host), sends the host $12 so he can buy the game. So everyone in the group only pays 1/5 the price. So if I where you i'd set up a paypal account to make things easier. Hope this helped.
PM or ask me if you have any other questions.