Here's a great warm-up activity perfect for large groups of any ability.
Have the players form a large circle. One player is designated Ringmaster and stands in the middle of the circle. The Ringmaster chooses a second player to stand in the middle with him/her.
Circled players take turns tossing a ball underhand, trying to tag the Ringmaster. Take care to aim chest level or lower. Ringmaster's second player's job is to keep the ball from hitting the Ringmaster. First player tagging Ringmaster is the new Ringmaster and selects a new guard to work with.
Require two or more tags before switching Ringmasters.
Allow two or more guards for the Ringmaster.
Adapted from ThePhysicalEducator.com