If you need to make sure someone sees your changes or comments, a mention is the best way to do it.
Here's how it works:
Any time you're editing text or writing a comment, type "@" followed by the person's name or email address.
Choose them from the popup that appears.
They'll get an email message with a link to the board you mentioned them in.
They'll also get a notification (in the app).
A few things to note:
This works in any type of card (notes, comments etc).
You can only mention people who are sharing the current board.