Basically, you are asking about creating a many to many relationship.
A client can relate to multiple users, and a usert ca relate to multiple clients.
To do this requires a linking table:
clientID - Primary Key
clientName - text
userID - Primary Key
UserName - text
ClientUsersID - Primary key
usersUserID - Foreign key relation to users.userID
clientsClientID - Foreign key relation to clients.clientID
the ClientUsers table above is used to create the many to many relationship of users to clients.
to have a check box that shows another form element, apply required client side validation to the checkbox on the onBlur event, then add the Show hide form element response from the client side validation, also on the on blur event.