You cannot buy tickets at the counter, but you have to "apply" for a spot. Usually this is what is meant with "registration". The organizers then either go "first come — first serve", or try to balance according to various criteria.

The "La Garúa Encuentro team"

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Dear milonguero friends,
Thank you for your enthousiasm for the La Garúa Encuentro. We received registrations from more than _ _ States and _ _Countries!
Now it is our job to guarantee a balance between experienced encuentro dancers and new ones, between the States, Countries, between leaders and followers...
Everyone will receive the result of our work in a personal Email before Monday September 14th. If for any reason, registrations are opened again, we will keep you posted.

The "La Garúa Encuentro team"