Swarms are open ended.

Agents choose to join the swarm when they think it's doing something useful that they want to be a part of.

What I've called a "positive boundary gradient" in the past.

People on the edge of the swarm would rather be in than out, which leads to the swarm growing.

Swarms tend to get momentum that becomes self-sustaining.

People who are aligned are more likely to stay and join.

People who are not aligned are more likely to leave or not join.

People who are somewhat aligned are more likely to become more aligned over time.