Add a message of the week / day / month header
The message header would be located at the top of all the bills OR pages, allowing for everyone to see the message at all times. When creating a message, it'll work similarly to the bills where you can set the predetermined time you want it to show. The other option here is to just have one box permanently stay up and have it edited as needed.
It could work similarly to the comments section where every time someone posted, it would post in a new box with their name attached to it. Or, it could just be a box where people, with permission, can edit as needed to say what they want to say. There should be something that automatically attaches their name to the sections they've edited as well, to keep track of who said what.
This would help ensure that if there are any comments that you've attached to the bill or images, that they're reminded to take a look at it. This could also help with communication as well.

We’d definitely like to add better comments that aren’t attached to expenses, especially for groups. Right now, there’s a rather hidden feature on our website called the “Whiteboard” (check the buttons at the top of the right sidebar), which is a little text box that any group member can edit. Maybe that can help you out for now while we work on better improvements!
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Ales Rybak commented
This would be very helpful. Or just make whiteboard more visible.