Well, saving some bucks is the point of the Pack. If it wouldn't be, then you could just use Premium as usual for your close family or friends.
The point is, it doesn't really matter if they're friends or family or not. If you have friends or family that are interested in using bunq, that's great! If not, there's always people like Mark trying to get people together. Who knows, maybe a new friendship can arise out of his efforts :)
Of course there is a small level of trust required to join a Pack with strangers. The organizer has to trust that the participants contribute their share of the monthly costs, and the participants have to trust that the organizer doesn't kick them out directly after receiving the payment. In either case, the worst that can happen if someone violates their trust is that the other party loses 5 euros.