[Steven Zahm]

@ Barbara

re: Is there a way for an admin to post messages just for the members to see?

Assuming members are registered WP users… sure, there are many plugin out there that allow you to post messages to your members (users). Maybe this one:
https://wordpress.org/plugins/popup-maker/

I never used it but it sounds like it moight have the feature you need.

re: Also how can I display a list of members by a certain category?

Yes, you could “hard code” a category to be shown by using the category shortcode option. Is that what you are asking?