How do you swarm in Pokemon Pearl?

You (as in the person) don't swarm. Instead, the Pokemon swarm. If you watch TV, sometimes they will announce that a bunch of _____ (a bunch of a certain Pokemon) has been spotted at ______ (a place). That is the swarm Pokemon. Swarm Pokemon are really special. Every day of the week there is a new swarm Pokemon. Catch them all!

Hope this helped! =)