Where can I find who has acces to my Wiki

If you click Settings in the upper right-hand corner of your wiki, and go to Users on the left-hand side, it will show you the people who you have allowed access to your wiki. If you have used the Invite Key though, you won't be able to see who logged in unless they also put in their name, and for that you'd be able to go to the Recent Activity tab of your Sidebar and see the last 10 visitors.

The best way to control who has access to your wiki is to have the people you want to see it sign up for PBwiki Accounts, and then have you go to Users again and add them as participant in your (free) wiki or as another level on your premium wiki. This way you can give people access without letting them delete pages or the whole wiki altogether.