Put a different colored dot on each desk in a group. When you need to have one student from each table do something, you can just simply say "Green dots, come get the papers for your table." or "Orange dots, take your table's trash to the trashcan."
Snowball Fight...kids write 3 things about themselves on a sheet of white paper. They do not write their names on the paper. Wad it up and on your command, have a "snowball" (or baseball) fight. At the end of 1 minute, each student grabs the nearest snowball and tries to guess who wrote it. Would be great at the beginning of the year!