You could create a group "all" which is private - hidden - no tags.
Every user is member of this group. So you cloud mention the group @all where all users are members.
I have selected the email verification option. user is successfully able to log in, but not receiving any verification mail. so no action can be done from the user side even if he logs in to account. Is it a problem of nodeBB version or any other mail configuration is required in the sandbox.
Have you set up an emailer yet?