You cannot prevent people staying in their preferred groups, that's generally impossible to do in 'big' meets...However, the leader should go around to each and every person and either introduce themself to newbies or say hello to people who have been met before...Meets are that hard to organise really... Set a time-frame to meet up of 30 minutes at a certain point, if people cannot make that time they can sort out something quickly after depending on their arrival... One of the team is determined as the group leader which could be different each trip and they're in charge of everyone... Go around together in the morning, call lunch at a certain time for an hour or so, meet at another specific point and then go around together for the rest of the day...Simples...