This is a fun way in which to break up a large group into smaller ones.
Write the names of characters from sitcoms, cartoons, etc. that can readily be recognized as belonging together on index cards. If you want your group size to be about 5, you should make sure that you have 5 characters per grouping.
For example, if you pick the Flintstones as a character grouping, you would have five index cards each with a different character such as Fred, Wilma, Barney, Pebbles, and Bamm-Bamm. Mix up the index cards and pass them out as the people walk through the door.
The groups then have to find each other by singing the theme song from whichever sitcom, cartoon, etc. the characters represent.