fieldmargin has a live messenger to discuss general goings on of your farm, useful for things that perhaps affect the whole farm or don’t relate to a specific place.
On web click the little voice bubbles in the top right and on mobile tap 'Farm Chat' in the hamburger menu to get chatting. You can use this to leave notes to yourself but it is most powerful when you are keeping everyone on your farm in the loop.
If you need to invite someone who’s not already on your farm the invite button is right there. You can find further instructions for inviting people to your farm here.
Type your message in the comment box at the bottom and then click send. On the mobile app, you can add in pictures to make something crystal clear by clicking the camera icon to the left of the comment bar. On the web app, you can upload all sorts of attachments, including pictures by clicking the paperclip icon to the left of the comment bar.