クライアント側のJavaScriptで、現在のユーザーがプライベートグループのメンバーかどうかを確認する方法はありますか？

[Falco]

For the current user, this will be done soon:

Plugin Development: List a user’s groups without making a request

@maja, can you add that to your list? We should always serialize all the group names of the current user (not only the primary group).

This will be enough for you to

@merefield

hide a javascript built advertising panel from site donators

and is the exact use case I had in mind.