You can see the name of the last person who made an edit at the bottom of the page and click on it to see details of the edit, including date and time. You can click on "complete history" to see all edits.
With many wikis, you set the parameters. These range from:
With many wikis, you can set it up to be as public or as private as you would like. You can open it up to everyone on the web or only invited members.
Again, in most cases, it depends on how the wiki is set up. You can set it up so that anyone can join or you can set it so that only people you invite may join.