IE shuts down when accessing CollectiveX

I have a set of clients who I have built a groupsite for - about 75 members. 15 of them cannot initially access the site when they accept their invitation and IE shuts down when they try. No error message - jsut shut down of the program. The others have had no problem. All are on the same network, no geographical similarities (e.g., from same part of US, etc.). They have tried over and over with no success and their IT department is getting concerned. Any ideas what may be causing this?