The games master instructs the group to break into smaller groups according to birthdays. Once in the birthday groups the group members introduce themselves to each other, and the largest group of same birthday scored 1 point for each group member.
Next the gamesmaster might call date of birthday and ask people to reform groups according to dates. You might want to introduce a 2 day either-side of date for this for smaller groups. If a person has a choice of two groups they get to choose which group to be a part of. Again the largest group scores 1 point per person.
Continue in this line asking groups to form and re-form according to your criteria. Each time, the group members introduce themselves to each other, and the largest group wins a point per person. Finally, when groups have had enough and enough introductions are made, the winners are the ones with the most points at the end.
Other criteria could be:
- Number of siblings
- Make up of siblings eg. 2 sisters or 1 sister and 2 brothers
- Place of birth
- Type of car or transport
- Age within 2 either side
- Name begins with...