You have to pitch tournament organizers. In exchange for a share of the savings tournament organizers mandate that teams use your service (could be hundreds of teams if it's a big tourney).

There are a lot of little companies doing exactly this, and tourney organizers are used to being pitched on it.

These little companies just put up a cheap Wordpress and use a service like Alliance on the backend, which is basically room blocking & invitations as-a-service. Not a particularly large barrier to entry. However, your site is by far the nicest & obviously has a lot of other advantages by virtue of YC.