Hi,
yes, you’ll only be able to create users on the main wiki.
The access only depends on the rights you put on the subwiki, they might get access to it if the wiki is open.
It really depends on your usecase, it’s hard to answer without knowing more what’s the target for your subwikis. In a farm like myxwiki.org for example we’re using global and local users: it allows to have admins defined at main wiki level and to let the admin of each subwiki to create / register their own user local to that subwiki.
Maybe the documentation in https://extensions.xwiki.org/xwiki/bin/view/Extension/Wiki%20Application#HHowtojoinawiki can help you. Now to be honest we’re internally wondering if that feature should be kept or not, see: Call for usecases around membership feature of subwikis
In the administration of the wiki you can add users after they are created.
I’m not sure we have a UI to edit them, as presented in the UI for creating the subwiki. Now the settings are stored in documents (I’d need to find which ones) so it’s definitely editable.