Site User List Web Part
Use this web part to gain a summary overview of who can use a site. When installed on a WSS or MOSS site it displays a list of all users who have permission to view that site. If you are interested in the technical details, you can read about the web part design here
How to Install this Free Download
Note - you will need page edit rights in the SharePoint site to be able to install this web part.
First download the file to your computer.
Open the SharePoint page on which you are going to install the web part,
open the Site Actions Menu on that page, and choose the "Edit Page" option.
Click on one of the "Add a Web Part" sections on the page. This will open
the Add Web Parts dialog window. Click on the "Advanced Web Part gallery and
options" link at the bottom right corner of this window.
An action pane opens on the right hand side of the window. Click on the
small arrow in the bar entitled "Browse", and select the "Import" option.
Use the Browse button in the form to find the web part file (named Site_User_List.dwp from inside the download web part ZIP file), and then click on the Upload button. When the Site User List will appear below the form, use the mouse to drag this item onto one of the web part zones on the page. When the web part is dropped onto a zone, the page will refresh and you will see the user list.
All that is left to do is click on the "Exit Edit Mode" link up by the Site Actions menu near the top right corner of the page.
To remove this web part, so into page edit mode, click on the "Edit" link in the title bar of the web part, and select "Delete".