As a Coach/Coachee/Mentor/Mentee, if you can't see a user, it means that their visibility settings are set to 'Hidden from all Users (except Administrators)' or 'Visible to Users only from my Organisation', and/or they have not ticked the box 'Users can message me'.
You may have to contact them outside the system and ask them to send a 'Connection request' to you. Or contact the Administrator for help. See 'How can I get support if I am having difficulties?'.
Champions will be able to see users from their own organisations and Administrators will be able to see users from all organisations, but both Champions and Administrators will only be able to message them individually if they have ticked the box 'Users can message me'.