One of the simplest approaches would be something like:
The idea of 'sending a message’ between users in an application is often just an illusion from the users perspective. From a software point of view it's often done by making the object visible to the different users based on different statuses.
As for examples, I think the basics for building your features is covered in Intermediate level (purple) Learning Paths in the Mendix Academy. If you work though those, you might get a better feeling for how to approach your challenge. It's been a while since I've done the learning paths, but maybe this one is of particular interest: Building an Event App Using Microflows