If the Groupsite Manager has set permissions so that all members can send Discussion Blasts, here is how you do it:
1. Click on the COMMUNICATE tab and select "Discussions"
2. Click on the name of the Forum where you want to create a new Discussion Topic
3. Click the "Create New Topic" box
4. Give you topic a title, write your post and before you hit "Save", check off the box that says "email the entire group immediately". That's it!