If you would like to attach a file when manually sending emails to clients or to your group members, you will first need to make sure you have already made the files available on the "Group Files" page.
Here's how:
Step 1. Follow the steps in this article: How do I use the “Group Files” section for my group?
Step 2. Now you are ready to attach the file to your email. Follow these steps: How do I send an email to my group?