You need a lot of tossable/catchable things. My favorites are sockballs and schmerltzes.
Get people into a circle. With 20 you might want to start with two or even three smaller circles.
Introduce one ball. Ask the group to toss the ball around the circle, being sure not to toss it to someone too close or too far away. And to continue until everyone gets the ball. (I usually ask people to hold up one hand until they've gotten the ball.) Then have them repeat the toss, exactly in the same pattern. After several rounds, introduce a second ball. (depending on competencies, the second ball can be in a new pattern or follow the established pattern). The task of the group is to see how many balls they can keep going without dropping any.
There are a veritable myriad ways to complicate this activity. Have them try to expand or contract the circle. Walk around. Change directions. Throw in reverse patterns. Add music, a handclapping rhythm.... The main dynamic is between how complex to make it and how to properly assess and engage the increasing skills of the group.
If it's not fun, it won't work, so keep it light and invite explorations, variations. Invite the different groups to demonstrate their final juggling routine to each other