In every work team there will be social cliques. These are groups of people who tend to hang out together at work, particularly when there's a social occasion. This is quite natural because humans are a social species, but it can mean that people do not mix with new people and this is one of the potential benefits of a team building event.
If you let teams form themselves then people will naturally gravitate towards their established cliques. If various departments are present then it is a good idea to choose teams in advance and purposefully so you have a balanced mix of people from each department.
This can be a worry for the organiser and they will often ask us, as a third party, to help out. While people may initially feel concerned that they are in a team of relative strangers we commonly hear feedback that this was one of the key benefits of the event. People may feel initial discomfort but, actually, they realise the benefits and enjoy meeting new people in a fun, relaxed situation. This can be extremely powerful for an organisation because it breaks down barriers between departments and encourages better communication.
Our Event Managers are very experienced in this situation and they will help the organiser by taking charge of team lists and making sure that people are in the right place. Having that external person in the mix helps by taking the pressure of people within the organisation.
Tip: If an external Event Manager is asked to pick teams it is important not to group people in batches, because they will tend to be standing in social cliques already. If you put people into teams by number as you face them (for example; 1, 2, 3, 4, 5 and then start again) then you will have the best chance of mixing people who do not know each other so well.