XP

The more the merrier. If every single participant bought every other participant a present, each additional participant would equate in an immense increase in the amount of presents.

If you celebrate alone, there won't be any presents under the tree. If there are two of you, there would be two presents under the tree. Three would equate to 6 presents and four would equate to 12. Each newcomer would receive a gift from the group and he/she would also need to buy presents for everyone.

And sometimes, a participant might prepare more than just one present for another participant. Thus there'll be an even larger increase in the amount of presents available.

So remember kids, more people = way more presents.