Anyone who has access to the Share link can add comments, as long as the creator has allowed them (see Sharing options article).
In that case, you can use the relevant switch, to make comments visible in the Player while previewing the prototype. You will be asked to identify yourself the first time. There are no anonymous comments.
You can add comments by clicking anywhere in, or out of the prototype. You can also reply to others’ comments. Comments are visible to all. You can optionally choose to notify other reviewers when you add comments.