Read credentials from web.config file

[ca]

I am trying to redirect a user to error page and display an error message
saying they do not have the proper credentials to access the applications in
vb.net. How do i do this at application level or website level. I want to
read the roles/group specified in web.config file and display it back to the
user saying you are not present in this group. But i want to do this
dynamically by reading/parsing the web.config file. I dont want to check and
see if the user is present in a certain group because i would not know the
group name before. Please let me know how to read the groups/roles specified
in web.config file.

Thanks for the help.