You'd likely want to look at using an external solution if you have a large number of clients, such as RADIUS, Active Directory or an external SSO provider.
I am not aware of any way to move these users over. If an organisation is split (i.e. split 1 or more networks out into their own organisation) Meraki Support can copy over organisation admins, but there's no way for us to move non-admin users (such as splash / meraki auth)