@dansup just let people have an option to specify the domain manually. If they already know that from somewhere then they most likely have a reason to use that specific instance.

It of course doesn't need to be shoved in your face and I personally would not include a full list of all known instances in the app directly but not offering signups on arbitrary instances as a feature to advanced/knowledgable users massively gimps the app and goes against the federated idea of the whole thing imo.