exchanging ssl cert keys?

hello all,

i have an Entrust SSL certificate installed through the Web Server Certificate Wizard. The idea is to have the client (an external party) for my web application authenticated by means of an SSL cert, and so i have configured the web site to use ssl communication and require client certificate. my question is: in order to map the client's certificate to be mapped to a windows account, what information do i need from the client about its certification, how do i obtain this information? similarly, what information would i be providing the client in order to access the client web application? does the certificate thumbprint have anything to do with this?

thanks in advance,
nc