I can see there is inbox and messages, how do they work?
The GymCube website has internal messaging which is much more powerful than the GymCube app, this is because a website offers more functionality than an app. As a client and coach, you can use internal messaging in the website to send basic messages back and forth as well as send/receive PDF's, Word docs, MP3's, MP4 videos. This is useful for keeping all your training content in one place, such as food diary's/exercise plans etc. for backdating, organisation of files and record keeping purposes.
The GymCube app also has internal messaging, but this has been implemented with communication of booking sessions in mind. You can use this to discuss online coaching and agree what you want to achieve before purchasing. You can also use it to coordinate your session times, e.g. perhaps you purchase 6 x 20-minute sessions, you may use the messaging feature to book your first session now with your online coach, and then use it again later when you need to book your other sessions etc. All you and the coach then have to do is show up on the app to connect with each other at the pre-agreed day and time (just like client and coach would have to do at the gym!).