In a nutshell it allows a group of a up-to five people to rock up at a bus stop, pay £5 and in return they get unlimited travel on National Express buses from 6pm to end of service.
That's just a £1 each for as many journeys as you want around the west midlands until closing time. A great reason for a group of mates to pop to the cinema, have a few beers or go to the bingo etc and know it isn't going to cost them a fortune for the travel.
The only caveat is that you have to travel together all the time for the ticket to be valid.
One can only assume that National Express are using this as a lost leader as they can't be making any money out of it. (Can they?) The bus is on a scheduled service anyway so you have to commend the operator for using it as a showcase for their product, without generating decent revenue. Buts surely it's better to have £5 and some positive word of mouth rather than nothing.
Incentivise people to try it and they might use it again for another reason. Or tell their friends.