Is there any option to send email in private to all members at once?
This has been discussed at length in this topic.
The recommended approach is to export your user list and import that into mailchimp or whatever mass mailing tool you prefer.
If you don’t want to use an external mass mailing tool, I’d suggest to create a category called “Newsletter” or “Updates and Announcements” or whatever and make all new users watch it by default (or better: watching first post). Existing users would have to be made to watch via command line in a one time operation.
As far as I know there is no built-in way of preventing users from un-watching that category, but that would also apply to Mailchimp & co.
Too bad I have no option to send private messages to all members, thank you for the attention.