Mass Add Group

I've set up an auto action to add new Registered users to a group called "Members". (I used onAfterNewUser) This works OK, but I want to add my 23000 users to the group as well. What trigger can I add for a 1 off action? I've tried a few including onAfterUserUpdate but it's not working.

You should be able to use the Batch Tools within CB > User Management to mass modify the usergroups of your users. It's also safe to use Joomlas batch tool for this in Users > Manage.

It'll will take some time as I can only select a max of 500 users at a time or the site crashes, as I have 23000+

There's not an efficient API method for doing what you're doing. Editing and storing that many user objects is a heavy operation. Fastest way would be to directly handle this in the database query queries. Your alternative is to handle giving them the usergroup on their next login using CB Auto Actions.