It is my understanding, based upon multiple communications with the company, that if the guest's name is placed on the reservation at the time that it is made, there is no need to thereafter supplement it with the form. The form is for changes to an existing reservation that has a different name on it, typically, the person who logged in to make the reservation.
If you log in with your individual account, your name is by default placed on the reservation. If you want to add your co-owner's name, do it at the time the initial reservation is being made. If you want to add a non-owner family member, add that name at the time that the reservation is made. If you do not do that, then submit the guest of owner form for a non-owner family member. I've not seen any discussion or guidance by the company that folks are required to go back and supplement if the name is already on the reservation.
There is a scenario when a co-owner isn't added to the reservation at the time it is made, and theoretically, that co-owner should be able to call to have their name placed on the reservation. That is what corporate says should happen, but due to the changes, CS seems confused and is telling folks that the Guest of Owner form needs to be used, regardless of whether or not the name being added for check in purposes is an existing co-owner. The company is supposedly working on educating CS in that regard. IDK how long that will take.