Ground staff can't always help. Check in agents are often under strict instructions from airlines NOT to move passengers that have pre booked and paid for seats together. In the summer a large percentage of charter passengers pre book their seats, which mean check in agents are left with only a few seats to play with. The family of four, turning up last to check in for a full flight, will simply have to be given the last four seats on the aeroplane, wherever these may be. Same family of four then turn up late to the gate, and expect other passengers to be moved to accomodate them. Understandably people who have paid for their seats are usually reluctant to move. Very often the majority of passengers will already be on board so cannot be asked to move seats by the ground staff. The only option then is to send passengers on board to see the crew. The crew have one advantage over us - they have the option of offering enticements such as free drinks to passengers who offer to move. Ground staff can't do that.
Even pre seating whole flights causes problems - someone who has been presat at the back wants to sit near the front, someone given a window wants an aisle, and groups of people travelling together who have not booked together want to sit together, but have been sat apart because they were under different bookings. They then complain they have been presat in seats they didn't want. You can't win!
If you want to sit together, bite the bullet and pay up, or make sure you are the first in line at check in. Why should people who have booked and paid or turned up early be expected to give up their seats for people who haven't?