I think the user list is only viewable to logged in used as default, but if not here is how to get there. First, go to your control panel. Click "Group Management" under "Forums" section in the left nav. Then on the line that is for "Guests" click "Site". Make sure that the first box "Can view the Member List" is set to 0. If not, then change it and hit the "Update this Group's Permissions" button at the bottom.

Now to edit the header click "Templates" under the "Display Options" section in the left nav. Once you are on the template editor page scroll down the list until you see header.tpl. Select it and then hit the "Edit Selected Template" button.

Copy the contents of the original template into notepad so that you'll have it in case something goes wrong.

I have one header that is already edited so this may not be the exact thing to look for but try searching for a line that has the following on it:
Code
<a href="{ubb url="ubb=showmembers"}">{$lang.USER_LIST}</a>
Then add your link on a new line below it.

Does that help? Feel free to ask more questions.

Also, realize that any changes you make to the templates will have to be reapplied whenever version 8 comes out. Make sure you keep a list of things to change whenever you upgrade.