email for a special group

I would like to get some assistance on how I can have a group of people accessing email in the DMZ where they can share information amongst themselves. They will be using OWA to access this server in the DMZ. They also want to use SSL to secure their emails. Would I have to do anything with MX record to give them access? Or do they need there own domain. I am not sure how to implement this solution. Can anybody tell me if they had a similar situation?