USTA QuickStart Workshop.
A number of balls is placed in the center of the court (near the T). The number of balls should be double the number of players. Balls should be contained somehow. I have seen hula hoops used, or buckets, hoppers, etc. Push comes to shove you can just have them lying on the court but in a circle or a mound to start off to make it clear these are the balls to be used in this activity and create a little visual interest.
This can be done individually or as a team. If as a team, one player can be designated as the 'keeper' protecting the team's supply of balls from other raiding teams.
This is a fun activity but as you can imagine there is a very good chance there will be some disgruntlement and even tears when players start robbing balls off each others' racquets. I have seen many players intent on defending their racquets after they get their three, very irate when others try to take them, and really hesitant to take balls from others' racquets. Important everyone understands it is okay to rob from other racquets. In fact, it is encouraged!
Number of balls you start with is unimportant, as long as it is lots of them. Player/team with most balls on their racquet at end of timed period wins.