App either has hard-coded pubkeys of trusted owners, or trusted "wot root" pubkeys, or may infer some context from the place where user is signing up. I.e. if user signs up on npub.pro site then we'll use site admin as wot root (we already reuse their relays for new profile, and suggest new user to follow them). Even if wot roots or owners are hard-coded I guess it's better then hardcoded signer pubkeys bcs new good signers can be discovered/suggested without recompiling the app.