User data segregation for multi-sites in Craft 3?
Craft 3 multi-site is an extension of locales in Craft 2, so how the data is stored is virtually identical.

The difference is that locales in Craft 2 are tied to language/country codes, but sites in Craft 3 are more abstract concepts that are user defined.


  If I have 'Site A' and 'Site B' running off the one install and someone creates an account on Site A, will they also have an account on Site B?


User accounts are global.  They are not created on a per-site basis, but they can be restricted on a per-site basis through user/user group permissions.


  Will we be able to restrict which CP users see data from which site?


User/user group have site permissions for elements that support it just like they have "locale" permissions in Craft 2.