Memberships + Internal Messaging?

Memberships + Internal Messaging?

I have some requirements for a site I'll be developing. The site must allow people to sign up and specify one or more categories (groups) to which they want to belong. Additionally, members must be able to send messages to each other through the system without exposing their email address - much like a PM system.

Can this currently be done with Zoho Sites, or would I need Creator or something other module or combination of modules? I'm new to Zoho and just learning about all the various apps and functionality, so any insight or direction would be appreciated!

-Steve