All you have to do is go to your "Find a borrower" page.
Here you will see the local borrowers in your area and the option to under each of their profile photos.
Click on the little group icon and then click to successfully add them to your group message.
The button will then show When you have selected all the borrowers you'd like to message go to the top right-hand corner of your search results page to see your messaging list. Click this to compose your message to all the borrowers you have chosen. You will see the names of the borrowers you are messaging too.