Hi Jackie,
Although there will be an airport representative to see you through check in, there is no set "meeting place" at Heathrow. When you arrive in Croatia you will all congregate in one place for the first time.
When you arrive at Heathrow, I would recommend making sure that you have one of the luggage tags on your hand luggage so fellow travellers can spot you and you can spot anyone else sporting one too - its a great way to break the ice.
Kindest regards,
Julia