On my Forum, when you log in as the Administrator, you can click on Members (top right of your screen) as per normal. However, you then have the option to Delete a member by clicking on the dustbin icon to the far right of that member's name.
I think the Forums are usually set up so that visitors "Add" themselves by clicking on the "Register" hyperlink.
Don't know about configuring private forums though.
. To delete a member/user; log in to your forum as the Administrator, click on the Members button/link on the top right corner of the screen. From here just find the user you want to delete and click on the dustbin/delete member button.
.A visitor can become a user by clicking on the Register button.
.You can create a forum and than make it private/password protect etc. You can choose which members can use which forum. All these options are available when you create a new forum.