How Chime knows the people of the app. The app writes one module, from the users it already has, and gives it to Chime's config:

defmodule MyApp.ChimeResolver do
  @behaviour Chime.Resolver

  import Ecto.Query

  alias MyApp.{Repo, User}

  @impl true
  def recipient(%User{} = user) do
    %Chime.Recipient{
      id: "user:#{user.id}",
      email: user.email,
      name: user.name,
      locale: user.locale,
      time_zone: user.time_zone
    }
  end

  @impl true
  def recipients(ids) do
    ids = for "user:" <> id <- ids, do: String.to_integer(id)

    from(u in User, where: u.id in ^ids and is_nil(u.deleted_at))
    |> Repo.all()
    |> Enum.map(&recipient/1)
  end

  @impl true
  def resolve_recipient(%Plug.Conn{assigns: %{current_user: %User{} = user}}),
    do: recipient(user)

  def resolve_recipient(%Phoenix.LiveView.Socket{assigns: %{current_user: %User{} = user}}),
    do: recipient(user)

  def resolve_recipient(_nobody), do: nil
end

recipient/1 turns what a notifier's recipients/1 gives (the app's users) into a Chime.Recipient. recipients/1 finds the people by their ids when a delivery goes, which may be later than the notification (an email ten minutes after, a digest the next morning), so that it writes to their address of then; a person it does not give back gets nothing, and the log says why. resolve_recipient/1 says who reads the feed: from a Plug.Conn (the JSON API, after the app's own pipeline) or a LiveView socket (Chime.Live, after the app's own on_mount hooks). Nil is nobody: the API answers 401, and the bell is not shown.

Make the ids from the database, never from what a browser sends.