wcf need suggestions

Question

I have a WPF application which will connect more than 100 users. Data fetching and storing everything based on my wcf service. this is working fine. *). I have enabled windows authentication for WCF. And this got some problems who are all in same domain they can access my services because this is windows authentication. *). To avoid this I have created group in IIS server which will allow only this group people. *). Again I got a problem if 5 to 10 users I can add manually but more than 100 users I can't add. Finally I came to know this group idea is not good. *). I need to enable proper security for WCF service. Only who is using my wpf application they should connect other user's they shouldn't connect. *). Could you please suggest how can I implement WCF service security? I'm new to WCF I don't have enough time to learn everything.