I am trying to define a list in Inform7 but it won't work.
Contacts is a list {"Melissa", "Paul"};
That does not work. Can you tell me how to define a list to which elements can be added?
You'll need to first define what kind of values the list holds before you can define what actual values it holds.
Contacts is a list of text that varies. Contacts is {"Melissa", "Paul"}.
This creates a global list that's accessible from anywhere. In rules and phrases you can create a list of constants (text, in this case) with a shorthand:
Instead of examining the phone book:
let contacts be {"Melissa", "Paul"};
...
(See also Writing with Inform §21.2.)