Well, I am not doing a popup since that requires code.
Everyone starting on the newb map is fine. It could be just like Steeltide newb island with several newb quests and a tutorial area. Then it would have an easy access warp stone off the map to the main map.
But yeah, multi-players on newb, just 1 allowed on main. Will be hard to really enforce though unless an admin on a lot. It just brings up all the issues we dealt with a decade ago with multis and it is like, in my head, who cares, have as many as you want. When you spread your time out, it is your players that suffer and never excel to higher levels.