I would prefer not to disrupt our current site. I want this to be a private site, what kind of control will we have? How about editing controls is that a function of the mediawiki software
You can start a new site with MediaWiki on a sub domain OR a newly registered domain. Doing this, your existing website won't be affected in any way. You should restrict editing of pages in your MediaWiki Site so that registered users can edit & view your Wiki content.
With MediaWiki hosting plans you get full access to manage the website. You can manage it with
1. MediaWiki Admin Panel
2. Website's Control Panel
Feel free to reply this thread if you have any more questions.