for the case your plugin gets activated after user-creations you'd have to use the hook action:plugin.activate (take care to compare the id you'll get with your plugins id since this fires for each plugin).
The following is the way I'd go within, maybe there is a more simple (or better performance) way:
Get somehow all user-ids (personally I don't know how, maybe this would help) and use the function setUserField of the user-module to set each users field-value.
Using action:user.create hook in a plugin to make the user a part of a specific group doesn't add the user to registered-users group
action:user.createhook in a plugin to make the user a part of a specific group doesn't add the user to registered-users group.
I had to call
groups.jointo make the user part of registered-users group.
By the time
action:user.createis fired the user should be already added to
registered-usersgroup, if not there is something terribly wrong
Thanks @baris. For some reason it didn't work for me when I started the plugin but it seems to work now.