Hi Yamuna,
By default the member list is visible to all authenticated users, that mean the user must be signed into the site to see the member list.
This setting in Web.config controls it:
<add key="RolesThatCanViewMemberList" value="Authenticated Users" />
if you want user who are not signed into the site to be able to see it then change it to:
<add key="RolesThatCanViewMemberList" value="All Users" />
ideally if you change this you should put the change in user.config so you won't lose this change when you upgrade later and get a enw Web.config
See Web.config Guide for more information about using auser.config file.
Hope it helps,
Joe