Congregate is a verb that means to come together, to assemble, or to gather. At school dances, you may congregate with your friends, because you get nervous in front of kids you don't know very well.
Congregate comes from Latin root words com, meaning "together" and gregare, meaning "to gather in a flock." Congregation, or a gathering or assembly, comes from the same roots, as does gregarious, an adjective describing someone who likes to gather with others to socialize. If you remember the key word is flock — what ministers often call their congregations — you should have no trouble remembering congregate means to flock together.