You just need to register a new account. It’s completely independent instance, it’s only federated with ani.social, which means you will see posts from ero.ani.social in global feed on ani.social and vice versa, but most other instances will only see post from ani.social in their global feed and not from ero.ani.social. It’s a great policy because people get easily offended by hentai and would easily go with crusade against whole instance if that was popping in their feed often. By the way, you can subscribe and post to ero.ani.social communities from ani.social as well, and you can find them through community search (there aren’t any at the moment, maybe I’ll create one as a test).