When a group of people gather, even if they are a close work team, they need some time to catch up and go through natural team bonding processes. This will be a familiar scene for anyone who has been on a team building event and it can be tricky for an instructor who has to break this natural process up to introduce an event. When you think about it, it's actually counter productive to do this. The group is doing something very natural and very powerful. If the aim of the day is team building then it's happening naturally.
You can structure a day to avoid losing out on this natural social team bonding. One way is to have a lunch first, or a breakfast if the session is in the morning. Buffet or canape style is more social, because people can move around and talk to more people in the room. With many events it's also preferable to eat a light meal before rather than something too heavy.
If a meal is not on the cards then consider building in half an hour before the event actually starts to allow people to catch up. It is important to put the start time of the event at the beginning of this half hour, rather than letting people know that the event actually starts later, because they will just turn up later.
Time is valuable, and it's not always available for this sort of planning. One of the ways that team building has changed in the last 10 years is that day long events have become half day events and there is now a trend to hold events at the end of the day, to minimise disruption to a business. However, if team building is the aim then it is important to understand how it happens around the organised activities as well as during them and to allow time for it to happen naturally.