This is a great question. Let me offer some background information. Aristophanes wrote many comedies in Athens and eleven of them survive today. Lysistrata is one of the most famous and it deals with the topic of the Peloponnesian War. The women are sick and tired of the protracted war. So, they concoct a plan to withhold any sexual relations until the war is over.
Now as for how she gets the women together, Aristophanes does not say. All we can say is that her friend, Calonice, was instrumental in helping her. In light of this, we can say that word of mouth was probably the way, which was the normal mode of communication in any ancient city during Aristophanes' time. We need to keep in mind that these cities were not enormous. When the women get together they all make an oath around a wine bowl and put their plan into practice.