Based on this you want to link one use to another in the form of a Post Relationship ? Unfortunately if the User's doesn't have a Profile in a custom post type then you won't be able to link it to their clients.
This is because Users can't have post relationships.
So you will need to have your Users create a profile under a Profile custom post type and Clients create a Profile under a Client Profile post type and from there they can be linked but not User -> User only Post -> Post.
User creates an account on our website
User creates a unique link and shares with a client
Client creates an account on our website over the unique link
Client is automatically linked to the user
This information is being displayed on both the user and the client Toolset form