Appalling service, left stranded

We had 8 tickets booked for the 6:40 flight & check in was at 6:55, we got there at 6:52 which was confirmed by the Safair consultant. We were then directed to the checkin queue. We waited in line & then was advised by the flight controller Lesego that the flight was closed, however he was scrambling around as if there was some confusion. We then had to pay an additional amount to be put on standby. The next flight was at 8:15 & checking was to close at 7:35 however while waiting for available seats for the flight checkin was still left open till 7:40. The flight controller then said that it was left open for the passengers in the queue as he went around asking who is still checking in. The only conclusion is that our seats were given away to other passengers due to overbooking. The staff are disgustingly rude & not to mention the poor management that seems to know nothing about customer service. I am appalled by the service & will not recommend anyone to use this airline. The only thing you get from them is I'm sorry, there’s nothing we can do for you ?