To let someone who is signed in be able to comment on a card, go to it's Options tab, click on the menu to the left of "...can comment on this card", select "Anyone signed in" and click "Save Changes".

• The commenting menu won't be in the Options tab if the card is controlled by a hard form, since the content of the card cannot be edited at all.
• If a card with commenting enabled is included in content or raw view then the commenting interface will not appear. So if you want it to appear include it with |open or |closed after the name.