I am the tree owner and of course can add people and their parents ...
I invited a person from my familly and once he sign in he can't add his parents.
I checked the edit option to make changes , why he should need to create a new familly for adding his parents
In fact if he create a new familly i will not be able to see his tree.
The only way i see actually is that i add his parents and then he will be able to make changes .