Existing chat groups have relied on Signal and Telegram. But 2025 has seen considerable expansions in Nostr related chat groups and 3rd party apps, such as White Noise, Session, SimpleX, BitChat, OPA, Keychat, etc.
Those 5 I just mentioned either use Nostr or will use Nostr or have no identity whatsoever. I personally would only support apps that support Nostr chats.