The ‘Discussions’ icon can be found both in the top right corner, as well as on the Athlete’s dashboard:
Click on the green + button to start a new discussion.
Complete all the required fields:
Only the invited users will be able to be part of this discussion.
General info: user functionalities depend on the roles & permissions settings. If a certain functionality does not work within your user account, this will be because you don't have sufficient permissions for this.