A: Non-seats/members aka "Collaborators" can also comment.
We really want you to share this with all your clients and collaborators as freely as possible.
The only requirement will be that they will need to signup (for free) after they attempt to leave a comment. Why? so they can receive email notifications of any followups to their comments.
When they attempt to comment on a screenshot, they will see this alert if they try to leave a new comment:
They will also see this link if they try to leave a new comment on an existing comment that you shared on the map:
Only team members can use PRIVATE comments, which won't appear publicly.