Use puzzle cards of three to create groups.
Buy kiddie cards from the Dollar Tree that use three cards to make a picture. I place one of the three cards on a table and hand out the other two cards randomly as students come in the room. When the time comes to pair up, students find their partner and then go to the table to make a picture (or find their matching table to meet their partner). Enrichment: You could use a puzzle to make larger groups as well!