What problems could "group bet" cause? Everyone makes his own decision to place a bet or not. Or your idea was that if a bet looses, one of the bettors will blame the other because he believed in his prediction or decision? It is a matter of trust then. Again it will be silly to blame someone for a loss. Personally I dont like the idea of a "group bet" because I see no benefit from that. The size of a bet does not impact on odds or win chance. Everyone still will receive same amount if everyone would have made an individual bet.
Betting in this way can cause some problems, one of the biggest ones is that in the case of a big win one of the players may claim the prize money and run away with it, something that it has happened many times on the past and it has even make the news, that being said, whenever we receive an offer like that from someone that is not a close person to us, the best thing we can do is to reject it, because the problems such a decision can carry do not outweigh the advantages we receive from it, that in this case are nonexistent.
Well, that the main concern that can also cause split in friendship or kinship, just because of certain amount of money from gambling wins, have to be willing to experience such problems.
But actually it is still very rare for one party to claim victory and run away with all the money because in betting with money collected everyone will definitely gather and wait together for the results to come out.
In my opinion, what often happens is in the division where there is one party who feels they deserve the most from the amount of victory, this will be debate that could possibly lead to fight.
Moreover, if this kind of bet is made with an offer to join people who are not known, then it can be much riskier, there will be something that might be much worse when victory is achieved and I agree with you that it is better to refuse such an offer.