I haven't implemented anything like this yet in Couch, but I believe the way to go about this would be using both the Member's module, as well as the 'Relationships' feature, described here:
http://www.couchcms.com/docs/concepts/r ... ships.htmlYou will have 'members', and also 'messages'. Each member is going to be 'related to' several messages, and each 'message' is going to be connected to (at least) 2 members (depending on how your
messaging is going to work).
Lastly, you will probably need to design your own interface for reading and sending messages - you may not even want your members to do that on the backend, preferring to build a frontend interface for the
messaging functions, or building the
messaging capabilities to already existing pages in your site. To do that, you'll need to use the "Databound Forms" features, described here:
http://www.couchcms.com/docs/concepts/d ... forms.htmlSo, in theory, you should be able to achieve what you're looking for, but as you can imagine, this is an advanced kind of setup, involving integrating many of the powerful features that Couch has to offer.
Don't let this scare you, though: it doesn't mean that it's going to be 'complicated', it just means that you'll have to be prepared to do a bit of reading, and spend some time trying out different things, before you find the solution that works for you. I would suggest you start with the basics of your site, and then move on to implementing each of these more advanced features as you need them, as slowly as possible - and not hesitating to come back here for help, when you come across a specific problem you can't solve!